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Investments (Details) $ in Thousands | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Feb. 29, 2016 | Sep. 30, 2016 USD ($) security | Sep. 30, 2015 USD ($) | Sep. 30, 2016 USD ($) security | Sep. 30, 2015 USD ($) | Dec. 31, 2015 USD ($) security
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | $ 4,609,551 |  | $ 4,609,551 |  | $ 4,689,773
Gross Unrealized Gains |  | 126,254 |  | 126,254 |  | 41,273
Gross Unrealized Losses |  | (9,962) |  | (9,962) |  | (67,840)
Fair Value |  | 4,725,843 |  | 4,725,843 |  | 4,663,206
Other-than-temporary impairments |  |  |  | 0 |  | 0
Federal Home Loan Bank advances, collateral, value of principal, percent (as a percent) | 102.00% |  |  |  |  |
FHLB advance collateral |  | 167,400 |  | 167,400 |  |
Gross accumulated unrealized gain (losses) on available-for-sale securities |  | $ (10,000) |  | $ (10,000) |  | $ (67,800)
Number of securities in unrealized loss position | security |  | 268 |  | 268 |  | 303
Net impairment losses recognized in earnings |  | $ 0 | $ 0 | $ 0 | $ 0 |
Total debt securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 4,602,457 |  | 4,602,457 |  | $ 4,684,148
Gross Unrealized Gains |  | 126,127 |  | 126,127 |  | 41,235
Gross Unrealized Losses |  | (9,959) |  | (9,959) |  | (67,822)
Fair Value |  | 4,718,625 |  | 4,718,625 |  | 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 |  | 41,735 |  | 41,735 |  | 160,393
Gross Unrealized Gains |  | 562 |  | 562 |  | 2,133
Gross Unrealized Losses |  | (41) |  | (41) |  | (1,942)
Fair Value |  | 42,256 |  | 42,256 |  | 160,584
Obligations of U.S. states and political subdivisions |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 2,046,199 |  | 2,046,199 |  | 1,766,407
Gross Unrealized Gains |  | 87,592 |  | 87,592 |  | 33,410
Gross Unrealized Losses |  | (1,122) |  | (1,122) |  | (7,290)
Fair Value |  | 2,132,669 |  | 2,132,669 |  | 1,792,527
Corporate debt securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 1,720,839 |  | 1,720,839 |  | 2,046,697
Gross Unrealized Gains |  | 33,409 |  | 33,409 |  | 2,836
Gross Unrealized Losses |  | (4,379) |  | (4,379) |  | (44,770)
Fair Value |  | 1,749,869 |  | 1,749,869 |  | 2,004,763
Asset-backed securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 79,032 |  | 79,032 |  | 116,764
Gross Unrealized Gains |  | 182 |  | 182 |  | 56
Gross Unrealized Losses |  | (32) |  | (32) |  | (203)
Fair Value |  | 79,182 |  | 79,182 |  | 116,617
Residential mortgage-backed securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 245,876 |  | 245,876 |  | 265,879
Gross Unrealized Gains |  | 616 |  | 616 |  | 161
Gross Unrealized Losses |  | (3,056) |  | (3,056) |  | (8,392)
Fair Value |  | 243,436 |  | 243,436 |  | 257,648
Commercial mortgage-backed securities |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 347,630 |  | 347,630 |  | 237,304
Gross Unrealized Gains |  | 3,563 |  | 3,563 |  | 162
Gross Unrealized Losses |  | (980) |  | (980) |  | (3,975)
Fair Value |  | 350,213 |  | 350,213 |  | 233,491
Collateralized loan obligations |  |  |  |  |  |
Schedule of Available-for-sale Securities [Line Items] |  |  |  |  |  |
Amortized Cost |  | 121,146 |  | 121,146 |  | 61,345
Gross Unrealized Gains |  | 203 |  | 203 |  | 3
Gross Unrealized Losses |  | (349) |  | (349) |  | (1,148)
Fair Value |  | 121,000 |  | 121,000 |  | 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 |  | 7,094 |  | 7,094 |  | 5,625
Gross Unrealized Gains |  | 127 |  | 127 |  | 38
Gross Unrealized Losses |  | (3) |  | (3) |  | (18)
Fair Value |  | $ 7,218 |  | $ 7,218 |  | $ 5,645

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