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Investments (Details)
$ in Thousands
1 Months Ended 3 Months Ended 6 Months Ended
Feb. 29, 2016
Jun. 30, 2016
USD ($)
security
Jun. 30, 2015
USD ($)
Jun. 30, 2016
USD ($)
security
Jun. 30, 2015
USD ($)
Dec. 31, 2015
USD ($)
security
Schedule of Available-for-sale Securities [Line Items]            
Amortized Cost   $ 4,427,591   $ 4,427,591   $ 4,689,773
Gross Unrealized Gains   151,912   151,912   41,273
Gross Unrealized Losses   (13,425)   (13,425)   (67,840)
Fair Value   4,566,078   4,566,078   4,663,206
Net impairment losses recognized in earnings   0 $ 0 0 $ 0  
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent) 102.00%          
FHLB advance collateral   166,600   166,600    
Gross accumulated unrealized gain (losses) on available-for-sale securities   $ (13,400)   $ (13,400)   $ (67,800)
Number of securities in unrealized loss position | security   211   211   303
Other-than-temporary impairments       $ 0 $ 0  
Total debt securities            
Schedule of Available-for-sale Securities [Line Items]            
Amortized Cost   $ 4,421,363   4,421,363   $ 4,684,148
Gross Unrealized Gains   148,261   148,261   41,235
Gross Unrealized Losses   (13,422)   (13,422)   (67,822)
Fair Value   4,556,202   4,556,202   4,657,561
U.S. Treasury securities and obligations of U.S. government corporations and agencies            
Schedule of Available-for-sale Securities [Line Items]            
Amortized Cost   59,860   59,860   160,393
Gross Unrealized Gains   2,276   2,276   2,133
Gross Unrealized Losses   (806)   (806)   (1,942)
Fair Value   61,330   61,330   160,584
Obligations of U.S. states and political subdivisions            
Schedule of Available-for-sale Securities [Line Items]            
Amortized Cost   1,968,194   1,968,194   1,766,407
Gross Unrealized Gains   109,789   109,789   33,410
Gross Unrealized Losses   (531)   (531)   (7,290)
Fair Value   2,077,452   2,077,452   1,792,527
Corporate debt securities            
Schedule of Available-for-sale Securities [Line Items]            
Amortized Cost   1,704,917   1,704,917   2,046,697
Gross Unrealized Gains   31,769   31,769   2,836
Gross Unrealized Losses   (6,721)   (6,721)   (44,770)
Fair Value   1,729,965   1,729,965   2,004,763
Asset-backed securities            
Schedule of Available-for-sale Securities [Line Items]            
Amortized Cost   104,221   104,221   116,764
Gross Unrealized Gains   236   236   56
Gross Unrealized Losses   (50)   (50)   (203)
Fair Value   104,407   104,407   116,617
Residential mortgage-backed securities            
Schedule of Available-for-sale Securities [Line Items]            
Amortized Cost   241,314   241,314   265,879
Gross Unrealized Gains   600   600   161
Gross Unrealized Losses   (3,574)   (3,574)   (8,392)
Fair Value   238,340   238,340   257,648
Commercial mortgage-backed securities            
Schedule of Available-for-sale Securities [Line Items]            
Amortized Cost   281,502   281,502   237,304
Gross Unrealized Gains   3,547   3,547   162
Gross Unrealized Losses   (891)   (891)   (3,975)
Fair Value   284,158   284,158   233,491
Collateralized loan obligations            
Schedule of Available-for-sale Securities [Line Items]            
Amortized Cost   61,355   61,355   61,345
Gross Unrealized Gains   44   44   3
Gross Unrealized Losses   (849)   (849)   (1,148)
Fair Value   60,550   60,550   60,200
Debt securities issued by foreign sovereign governments            
Schedule of Available-for-sale Securities [Line Items]            
Amortized Cost           29,359
Gross Unrealized Gains           2,474
Gross Unrealized Losses           (102)
Fair Value           31,731
Equity securities            
Schedule of Available-for-sale Securities [Line Items]            
Amortized Cost   6,228   6,228   5,625
Gross Unrealized Gains   3,651   3,651   38
Gross Unrealized Losses   (3)   (3)   (18)
Fair Value   $ 9,876   $ 9,876   $ 5,645