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Securities Available for Sale and Held to Maturity - Securities with Gross Unrealized Losses, Segregated by Length of Impairment (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Estimated Fair Value, Less than 12 Months $ 1,910,791 $ 2,033,477
Estimated Fair Value, 12 Months or More 108,607 114,975
Estimated Fair Value, Total 2,019,398 2,148,452
Gross Unrealized (Losses), Less than 12 Months (23,153) (26,714)
Gross Unrealized (Losses), 12 Months or More (3,556) (3,793)
Gross Unrealized (Losses), Total (26,709) (30,507)
U.S. Government Agency Obligations Issued by U.S. Government Agencies [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Estimated Fair Value, Less than 12 Months 6,941 9,420
Estimated Fair Value, 12 Months or More 34,020 33,248
Estimated Fair Value, Total 40,961 42,668
Gross Unrealized (Losses), Less than 12 Months (102) (142)
Gross Unrealized (Losses), 12 Months or More (727) (783)
Gross Unrealized (Losses), Total (829) (925)
Obligations of States and Political Subdivisions [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Estimated Fair Value, Less than 12 Months 9,551 20,539
Estimated Fair Value, 12 Months or More 652 654
Estimated Fair Value, Total 10,203 21,193
Gross Unrealized (Losses), Less than 12 Months (44) (135)
Gross Unrealized (Losses), 12 Months or More (4) (5)
Gross Unrealized (Losses), Total (48) (140)
Residential Mortgage Pass-Through Securities Guaranteed by GNMA [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Estimated Fair Value, Less than 12 Months 42,618 43,615
Estimated Fair Value, 12 Months or More 212 222
Estimated Fair Value, Total 42,830 43,837
Gross Unrealized (Losses), Less than 12 Months (964) (822)
Gross Unrealized (Losses), 12 Months or More (2) (6)
Gross Unrealized (Losses), Total (966) (828)
Residential Mortgage Pass-Through Securities Issued by FNMA and FHLMC [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Estimated Fair Value, Less than 12 Months 598,901 588,352
Estimated Fair Value, 12 Months or More 46 0
Estimated Fair Value, Total 598,947 588,352
Gross Unrealized (Losses), Less than 12 Months (8,006) (10,582)
Gross Unrealized (Losses), 12 Months or More (1) 0
Gross Unrealized (Losses), Total (8,007) (10,582)
Other Residential Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Estimated Fair Value, Less than 12 Months 1,067,305 1,127,501
Estimated Fair Value, 12 Months or More 72,596 76,196
Estimated Fair Value, Total 1,139,901 1,203,697
Gross Unrealized (Losses), Less than 12 Months (11,926) (12,722)
Gross Unrealized (Losses), 12 Months or More (2,821) (2,909)
Gross Unrealized (Losses), Total (14,747) (15,631)
Commercial Mortgage-Backed Securities Issued or Guaranteed by FNMA, FHLMC or GNMA [Member]    
Schedule of Available For Sale and Held to Maturity Securities [Line Items]    
Estimated Fair Value, Less than 12 Months 185,475 244,050
Estimated Fair Value, 12 Months or More 1,081 4,655
Estimated Fair Value, Total 186,556 248,705
Gross Unrealized (Losses), Less than 12 Months (2,111) (2,311)
Gross Unrealized (Losses), 12 Months or More (1) (90)
Gross Unrealized (Losses), Total $ (2,112) $ (2,401)