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Available for Sale Securities (Details)
3 Months Ended 12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost $ 8,952,391,000 $ 8,952,391,000 $ 8,369,075,000  
Available for sale securities, Fair Value 8,857,120,000 8,857,120,000 8,321,578,000  
Available for sale securities, Gross Unrealized Gain 42,986,000 42,986,000 41,519,000  
Available for sale securities, Gross Unrealized Loss 138,257,000 138,257,000 88,625,000  
Available for sale securities, OTTI 0 0 (391,000)  
Available for sale securities, Pledged as Collateral, Fair Value $ 9,100,000,000 $ 9,100,000,000 7,300,000,000  
Available for Sale Securities, Debt Maturities, Nominal Yield [Abstract]        
Available for Sale Securities, Debt Maturities, Nominal Yield 2.37% 2.37%    
Available for Sale Securities, Realized Gain (Loss) [Abstract]        
Proceeds from sales of available for sale securities   $ 745,643,000 1,309,215,000 $ 899,381,000
Available for Sale Securities, Gross realized gains   7,117,000 10,223,000 11,696,000
Available for Sale Securities, Gross realized losses   (9,918,000) (5,795,000) (21,000)
Available for Sale Securities, Related federal and state income tax expense   $ (713,000) $ 1,722,000 $ 4,542,000
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions 490 490 534  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months $ 700,064,000 $ 700,064,000 $ 3,773,942,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer 5,631,803,000 5,631,803,000 2,118,360,000  
Available for Sale Securities, Unrealized Loss Position 6,331,867,000 6,331,867,000 5,892,302,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1,615,000 1,615,000 30,648,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 136,642,000 136,642,000 58,368,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss 138,257,000 138,257,000 89,016,000  
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net [Abstract]        
Other-than-temporary Impairment Loss, Available for Sale Securities, Recognized in Earnings   0 0  
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Cumulative Credit Losses on Debt Securities Held 45,000,000 45,000,000 55,000,000  
Fixed maturity securities [Member]        
Available for Sale Securities, Debt Maturities, Amortized Cost, Rolling Maturity [Abstract]        
Available for Sale Securities, Debt Maturities, Less than One Year, Amortized Cost 78,615,000 78,615,000    
Available for Sale Securities, Debt Maturities, One to Five Years, Amortized Cost 1,109,788,000 1,109,788,000    
Available for Sale Securities, Debt Maturities, Six to Ten Years, Amortized Cost 1,497,468,000 1,497,468,000    
Available for Sale Securities, Debt Maturities, Over Ten Years, Amortized Cost 339,249,000 339,249,000    
Available for Sale Securities, Maturity, Allocated and Single Maturity Date, Amortized Cost 3,025,120,000 3,025,120,000    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]        
Available for Sale Securities, Debt Maturities, Less Than One Year, Fair Value 77,965,000 77,965,000    
Available for Sale Securities, Debt Maturities, One to Five Years, Fair Value 1,091,285,000 1,091,285,000    
Available for Sale Securities, Debt Maturities, Six to Ten Years, Fair Value 1,486,939,000 1,486,939,000    
Available for Sale Securities, Debt Maturities, Over Ten Years, Fair Value 336,487,000 336,487,000    
Available for Sale Securities, Debt Maturities, Single Maturity Date, Fair value $ 2,992,676,000 $ 2,992,676,000    
Available for Sale Securities, Debt Maturities, Nominal Yield [Abstract]        
Available for Sale Securities, Debt Maturities, Less than one year, Nominal Yield 1.73% 1.73%    
Available for Sale Securities, Debt Maturities, One to five years, Nominal Yield 2.07% 2.07%    
Available for Sale Securities, Debt Maturities, Six to ten years, Nominal Yield 2.44% 2.44%    
Available for Sale Securities, Debt Maturities, Over ten years, Nominal Yield 2.48% 2.48%    
Available for Sale Securities, Debt Maturities, Nominal Yield 2.29% 2.29%    
Available for Sale Securities, Debt Maturities, Weighted Average Maturity [1] 6.98 6.98    
U.S. Treasury [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost $ 496,000 $ 496,000 1,000,000  
Available for sale securities, Fair Value 493,000 493,000 1,000,000  
Available for sale securities, Gross Unrealized Gain 0 0 0  
Available for sale securities, Gross Unrealized Loss 3,000 3,000 0  
Available for sale securities, OTTI 0 0 $ 0  
Available for Sale Securities, Debt Maturities, Amortized Cost, Rolling Maturity [Abstract]        
Available for Sale Securities, Debt Maturities, Less than One Year, Amortized Cost 0 0    
Available for Sale Securities, Debt Maturities, One to Five Years, Amortized Cost 496,000 496,000    
Available for Sale Securities, Debt Maturities, Six to Ten Years, Amortized Cost 0 0    
Available for Sale Securities, Debt Maturities, Over Ten Years, Amortized Cost 0 0    
Available for Sale Securities, Maturity, Allocated and Single Maturity Date, Amortized Cost 496,000 496,000    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]        
Available for Sale Securities, Debt Maturities, Less Than One Year, Fair Value 0 0    
Available for Sale Securities, Debt Maturities, One to Five Years, Fair Value 493,000 493,000    
Available for Sale Securities, Debt Maturities, Six to Ten Years, Fair Value 0 0    
Available for Sale Securities, Debt Maturities, Over Ten Years, Fair Value 0 0    
Available for Sale Securities, Debt Maturities, Single Maturity Date, Fair value $ 493,000 $ 493,000    
Available for Sale Securities, Debt Maturities, Nominal Yield [Abstract]        
Available for Sale Securities, Debt Maturities, Less than one year, Nominal Yield 0.00% 0.00%    
Available for Sale Securities, Debt Maturities, One to five years, Nominal Yield 1.99% 1.99%    
Available for Sale Securities, Debt Maturities, Six to ten years, Nominal Yield 0.00% 0.00%    
Available for Sale Securities, Debt Maturities, Over ten years, Nominal Yield 0.00% 0.00%    
Available for Sale Securities, Debt Maturities, Nominal Yield 1.99% 1.99%    
Available for Sale Securities, Debt Maturities, Weighted Average Maturity [1] 1.08 1.08    
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions 1 1 0  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months $ 0 $ 0 $ 0  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer 493,000 493,000 0  
Available for Sale Securities, Unrealized Loss Position 493,000 493,000 0  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0 0  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 3,000 3,000 0  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss 3,000 3,000 0  
Municipal and other tax-exempt securities [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost 2,782,000 2,782,000 27,182,000  
Available for sale securities, Fair Value 2,864,000 2,864,000 27,080,000  
Available for sale securities, Gross Unrealized Gain 82,000 82,000 181,000  
Available for sale securities, Gross Unrealized Loss 0 0 283,000  
Available for sale securities, OTTI 0 0 $ 0  
Available for Sale Securities, Debt Maturities, Amortized Cost, Rolling Maturity [Abstract]        
Available for Sale Securities, Debt Maturities, Less than One Year, Amortized Cost 1,057,000 1,057,000    
Available for Sale Securities, Debt Maturities, One to Five Years, Amortized Cost 1,725,000 1,725,000    
Available for Sale Securities, Debt Maturities, Six to Ten Years, Amortized Cost 0 0    
Available for Sale Securities, Debt Maturities, Over Ten Years, Amortized Cost 0 0    
Available for Sale Securities, Maturity, Allocated and Single Maturity Date, Amortized Cost 2,782,000 2,782,000    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]        
Available for Sale Securities, Debt Maturities, Less Than One Year, Fair Value 1,063,000 1,063,000    
Available for Sale Securities, Debt Maturities, One to Five Years, Fair Value 1,801,000 1,801,000    
Available for Sale Securities, Debt Maturities, Six to Ten Years, Fair Value 0 0    
Available for Sale Securities, Debt Maturities, Over Ten Years, Fair Value 0 0    
Available for Sale Securities, Debt Maturities, Single Maturity Date, Fair value $ 2,864,000 $ 2,864,000    
Available for Sale Securities, Debt Maturities, Nominal Yield [Abstract]        
Available for Sale Securities, Debt Maturities, Less than one year, Nominal Yield [2] 6.69% 6.69%    
Available for Sale Securities, Debt Maturities, One to five years, Nominal Yield [2] 6.45% 6.45%    
Available for Sale Securities, Debt Maturities, Six to ten years, Nominal Yield [2] 0.00% 0.00%    
Available for Sale Securities, Debt Maturities, Over ten years, Nominal Yield [2] 0.00% 0.00%    
Available for Sale Securities, Debt Maturities, Nominal Yield [2] 6.54% 6.54%    
Available for Sale Securities, Debt Maturities, Weighted Average Maturity [1] 1.67 1.67    
Available for Sale Securities, Debt Maturities, Effective tax rate for nominal yield calculation 25.00% 25.00%    
Available for Sale Securities, Debt Maturities, Number of days for interest rates to reset 35 days      
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions 0 0 19  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months $ 0 $ 0 $ 12,765,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer 0 0 4,802,000  
Available for Sale Securities, Unrealized Loss Position 0 0 17,567,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 0 18,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 0 265,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss 0 0 283,000  
Residential Mortgage Backed Securities [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost 5,927,271,000 5,927,271,000 5,429,459,000  
Available for sale securities, Fair Value 5,864,444,000 5,864,444,000 5,402,373,000  
Available for sale securities, Gross Unrealized Gain 34,937,000 34,937,000 35,605,000  
Available for sale securities, Gross Unrealized Loss 97,764,000 97,764,000 62,300,000  
Available for sale securities, OTTI 0 0 $ (391,000)  
Available for Sale Securities, Debt Maturities, Amortized Cost, Rolling Maturity [Abstract]        
Available for Sale Securities, Maturity, without Single Maturity Date, Amortized Cost 5,927,271,000 5,927,271,000    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]        
Available for Sale Securities, Debt Maturities, without Single Maturity Date, Fair value $ 5,864,444,000 $ 5,864,444,000    
Available for Sale Securities, Debt Maturities, Nominal Yield [Abstract]        
Available for Sale Securities, Debt Maturities, Nominal Yield [3] 2.41% 2.41%    
Available for Sale Securities, Debt Maturities, Average Expected Life of Mortgage-backed Securities   4 years 1 month 6 days    
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions 289 289 217  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months $ 510,824,000 $ 510,824,000 $ 2,274,604,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer 3,641,370,000 3,641,370,000 1,458,587,000  
Available for Sale Securities, Unrealized Loss Position 4,152,194,000 4,152,194,000 3,733,191,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1,158,000 1,158,000 18,758,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 96,606,000 96,606,000 43,933,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss 97,764,000 97,764,000 62,691,000  
U.S. government agency residential mortgage-backed securities [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost 5,886,323,000 5,886,323,000 5,355,148,000  
Available for sale securities, Fair Value 5,804,708,000 5,804,708,000 5,309,152,000  
Available for sale securities, Gross Unrealized Gain 16,149,000 16,149,000 16,304,000  
Available for sale securities, Gross Unrealized Loss 97,764,000 97,764,000 62,300,000  
Available for sale securities, OTTI $ 0 $ 0 $ 0  
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions 289 289 209  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months $ 510,824,000 $ 510,824,000 $ 2,268,706,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer 3,641,370,000 3,641,370,000 1,458,587,000  
Available for Sale Securities, Unrealized Loss Position 4,152,194,000 4,152,194,000 3,727,293,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 1,158,000 1,158,000 18,367,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 96,606,000 96,606,000 43,933,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss 97,764,000 97,764,000 62,300,000  
U.S. government agency residential mortgage-backed securities [Member] | FNMA [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost 3,414,573,000 3,414,573,000 3,021,551,000  
Available for sale securities, Fair Value 3,367,124,000 3,367,124,000 2,997,563,000  
Available for sale securities, Gross Unrealized Gain 10,559,000 10,559,000 11,549,000  
Available for sale securities, Gross Unrealized Loss 58,008,000 58,008,000 35,537,000  
Available for sale securities, OTTI $ 0 $ 0 $ 0  
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions 162 162 113  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months $ 161,089,000 $ 161,089,000 $ 1,203,041,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer 2,135,377,000 2,135,377,000 824,029,000  
Available for Sale Securities, Unrealized Loss Position 2,296,466,000 2,296,466,000 2,027,070,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 542,000 542,000 9,618,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 57,466,000 57,466,000 25,919,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss 58,008,000 58,008,000 35,537,000  
U.S. government agency residential mortgage-backed securities [Member] | FHLMC [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost 1,723,399,000 1,723,399,000 1,545,971,000  
Available for sale securities, Fair Value 1,699,779,000 1,699,779,000 1,531,009,000  
Available for sale securities, Gross Unrealized Gain 5,189,000 5,189,000 3,148,000  
Available for sale securities, Gross Unrealized Loss 28,809,000 28,809,000 18,110,000  
Available for sale securities, OTTI $ 0 $ 0 $ 0  
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions 85 85 69  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months $ 71,205,000 $ 71,205,000 $ 863,778,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer 1,129,730,000 1,129,730,000 385,816,000  
Available for Sale Securities, Unrealized Loss Position 1,200,935,000 1,200,935,000 1,249,594,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 328,000 328,000 7,297,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 28,481,000 28,481,000 10,813,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss 28,809,000 28,809,000 18,110,000  
U.S. government agency residential mortgage-backed securities [Member] | GNMA [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost 748,351,000 748,351,000 787,626,000  
Available for sale securities, Fair Value 737,805,000 737,805,000 780,580,000  
Available for sale securities, Gross Unrealized Gain 401,000 401,000 1,607,000  
Available for sale securities, Gross Unrealized Loss 10,947,000 10,947,000 8,653,000  
Available for sale securities, OTTI $ 0 $ 0 $ 0  
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions 42 42 27  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months $ 278,530,000 $ 278,530,000 $ 201,887,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer 376,263,000 376,263,000 248,742,000  
Available for Sale Securities, Unrealized Loss Position 654,793,000 654,793,000 450,629,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 288,000 288,000 1,452,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 10,659,000 10,659,000 7,201,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss 10,947,000 10,947,000 8,653,000  
Privately issued residential mortgage-backed securities [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost 40,948,000 40,948,000 74,311,000  
Available for sale securities, Fair Value 59,736,000 59,736,000 93,221,000  
Available for sale securities, Gross Unrealized Gain 18,788,000 18,788,000 19,301,000  
Available for sale securities, Gross Unrealized Loss 0 0 0  
Available for sale securities, OTTI $ 0 $ 0 $ (391,000)  
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions 0 [4] 0 [4] 8 [5]  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months $ 0 [4] $ 0 [4] $ 5,898,000 [5]  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer 0 [4] 0 [4] 0 [5]  
Available for Sale Securities, Unrealized Loss Position 0 [4] 0 [4] 5,898,000 [5]  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 0 [4] 0 [4] 391,000 [5]  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 0 [4] 0 [4] 0 [5]  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss 0 [4] 0 [4] 391,000 [5]  
Commercial mortgage-backed securities guaranteed by U.S. government agencies [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost 2,986,297,000 2,986,297,000 2,858,885,000  
Available for sale securities, Fair Value 2,953,889,000 2,953,889,000 2,834,961,000  
Available for sale securities, Gross Unrealized Gain 7,955,000 7,955,000 1,963,000  
Available for sale securities, Gross Unrealized Loss 40,363,000 40,363,000 25,887,000  
Available for sale securities, OTTI 0 0 $ 0  
Available for Sale Securities, Debt Maturities, Amortized Cost, Rolling Maturity [Abstract]        
Available for Sale Securities, Debt Maturities, Less than One Year, Amortized Cost 77,558,000 77,558,000    
Available for Sale Securities, Debt Maturities, One to Five Years, Amortized Cost 1,107,567,000 1,107,567,000    
Available for Sale Securities, Debt Maturities, Six to Ten Years, Amortized Cost 1,497,468,000 1,497,468,000    
Available for Sale Securities, Debt Maturities, Over Ten Years, Amortized Cost 303,704,000 303,704,000    
Available for Sale Securities, Maturity, Allocated and Single Maturity Date, Amortized Cost 2,986,297,000 2,986,297,000    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]        
Available for Sale Securities, Debt Maturities, Less Than One Year, Fair Value 76,902,000 76,902,000    
Available for Sale Securities, Debt Maturities, One to Five Years, Fair Value 1,088,991,000 1,088,991,000    
Available for Sale Securities, Debt Maturities, Six to Ten Years, Fair Value 1,486,939,000 1,486,939,000    
Available for Sale Securities, Debt Maturities, Over Ten Years, Fair Value 301,057,000 301,057,000    
Available for Sale Securities, Debt Maturities, Single Maturity Date, Fair value $ 2,953,889,000 $ 2,953,889,000    
Available for Sale Securities, Debt Maturities, Nominal Yield [Abstract]        
Available for Sale Securities, Debt Maturities, Less than one year, Nominal Yield 1.66% 1.66%    
Available for Sale Securities, Debt Maturities, One to five years, Nominal Yield 2.06% 2.06%    
Available for Sale Securities, Debt Maturities, Six to ten years, Nominal Yield 2.44% 2.44%    
Available for Sale Securities, Debt Maturities, Over ten years, Nominal Yield 2.54% 2.54%    
Available for Sale Securities, Debt Maturities, Nominal Yield 2.29% 2.29%    
Available for Sale Securities, Debt Maturities, Weighted Average Maturity [1] 6.90 6.90    
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions 197 197 185  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months $ 179,258,000 $ 179,258,000 $ 1,465,703,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer 1,969,504,000 1,969,504,000 652,296,000  
Available for Sale Securities, Unrealized Loss Position 2,148,762,000 2,148,762,000 2,117,999,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 394,000 394,000 11,824,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 39,969,000 39,969,000 14,063,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss 40,363,000 40,363,000 25,887,000  
Perpetual preferred stock [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost     12,562,000  
Available for sale securities, Fair Value     15,767,000  
Available for sale securities, Gross Unrealized Gain     3,205,000  
Available for sale securities, Gross Unrealized Loss     0  
Available for sale securities, OTTI     $ 0  
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions     0  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months     $ 0  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer     0  
Available for Sale Securities, Unrealized Loss Position     0  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss     0  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss     0  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss     0  
Equity securities and mutual funds [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost     14,487,000  
Available for sale securities, Fair Value     14,916,000  
Available for sale securities, Gross Unrealized Gain     515,000  
Available for sale securities, Gross Unrealized Loss     86,000  
Available for sale securities, OTTI     $ 0  
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions     111  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months     $ 911,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer     2,203,000  
Available for Sale Securities, Unrealized Loss Position     3,114,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss     7,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss     79,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss     86,000  
Other debt securities [Member]        
Available for Sale Securities [Line Items]        
Available for sale securities, Amortized Cost 35,545,000 35,545,000 25,500,000  
Available for sale securities, Fair Value 35,430,000 35,430,000 25,481,000  
Available for sale securities, Gross Unrealized Gain 12,000 12,000 50,000  
Available for sale securities, Gross Unrealized Loss 127,000 127,000 69,000  
Available for sale securities, OTTI 0 0 $ 0  
Available for Sale Securities, Debt Maturities, Amortized Cost, Rolling Maturity [Abstract]        
Available for Sale Securities, Debt Maturities, Less than One Year, Amortized Cost 0 0    
Available for Sale Securities, Debt Maturities, One to Five Years, Amortized Cost 0 0    
Available for Sale Securities, Debt Maturities, Six to Ten Years, Amortized Cost 0 0    
Available for Sale Securities, Debt Maturities, Over Ten Years, Amortized Cost 35,545,000 35,545,000    
Available for Sale Securities, Maturity, Allocated and Single Maturity Date, Amortized Cost 35,545,000 35,545,000    
Available for Sale Securities, Maturity, Fair Value, Rolling Maturity [Abstract]        
Available for Sale Securities, Debt Maturities, Less Than One Year, Fair Value 0 0    
Available for Sale Securities, Debt Maturities, One to Five Years, Fair Value 0 0    
Available for Sale Securities, Debt Maturities, Six to Ten Years, Fair Value 0 0    
Available for Sale Securities, Debt Maturities, Over Ten Years, Fair Value 35,430,000 35,430,000    
Available for Sale Securities, Debt Maturities, Single Maturity Date, Fair value $ 35,430,000 $ 35,430,000    
Available for Sale Securities, Debt Maturities, Nominal Yield [Abstract]        
Available for Sale Securities, Debt Maturities, Less than one year, Nominal Yield 0.00% 0.00%    
Available for Sale Securities, Debt Maturities, One to five years, Nominal Yield 0.00% 0.00%    
Available for Sale Securities, Debt Maturities, Six to ten years, Nominal Yield 0.00% 0.00%    
Available for Sale Securities, Debt Maturities, Over ten years, Nominal Yield [6] 1.94% 1.94%    
Available for Sale Securities, Debt Maturities, Nominal Yield 1.94% 1.94%    
Available for Sale Securities, Debt Maturities, Weighted Average Maturity [1] 13.61 13.61    
Available for Sale Securities, Continuous Unrealized Loss Position, Qualitative Disclosure [Abstract]        
Available for Sale Securities, Unrealized Loss Position, Number of Positions 3 3 2  
Available for Sale Securities, Unrealized Loss Position [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months $ 9,982,000 $ 9,982,000 $ 19,959,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer 20,436,000 20,436,000 472,000  
Available for Sale Securities, Unrealized Loss Position 30,418,000 30,418,000 20,431,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss [Abstract]        
Available for Sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss 63,000 63,000 41,000  
Available for Sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss 64,000 64,000 28,000  
Available for Sale Securities, Unrealized Loss Position, Accumulated Loss $ 127,000 $ 127,000 $ 69,000  
[1] Expected maturities may differ from contractual maturities, because borrowers may have the right to call or prepay obligations with or without penalty.
[2] Calculated on a taxable equivalent basis using a 25 percent effective tax rate.
[3] The nominal yield on mortgage-backed securities is based upon prepayment assumptions at the purchase date. Actual yields earned may differ significantly based upon actual prepayments. See Quarterly Financial Summary –– Unaudited following for current yields on available for sale securities portfolio.
[4] Includes securities for which an unrealized loss remains in AOCI after an other-than-temporary credit loss has been recognized in income.
[5] Includes securities for which an unrealized loss remains in AOCI after an other-than-temporary credit loss has been recognized in income.
[6] Nominal yield on other debt securities with contractual maturity dates over ten years are based on variable rates which generally are reset within 35 days.