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Investments (Tables) | 12 Months Ended
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Dec. 31, 2017
Investment [Line Items] |
Schedule of Unrealized Gains On Investments |
($ in thousands) |  | 2017 |  | 2016 |  | 2015
AFS securities: |  |  |  |  |  |  |  |  |
Fixed income securities |  | $ | 85,806 |  |  | 38,781 |  |  | 55,689 |
Equity securities |  | 38,894 |  |  | 25,864 |  |  | 13,235 |
Total AFS securities |  | 124,700 |  |  | 64,645 |  |  | 68,924 |
HTM securities: |  |  |  |  |  |  |  |  |
Fixed income securities |  | (21 | ) |  | 159 |  |  | 300 |
Total HTM securities |  | (21 | ) |  | 159 |  |  | 300 |
Total net unrealized gains |  | 124,679 |  |  | 64,804 |  |  | 69,224 |
Deferred income tax |  | (44,103 | ) |  | (22,681 | ) |  | (24,228 | )
Net unrealized gains, net of deferred income tax |  | 80,576 |  |  | 42,123 |  |  | 44,996 |
Increase (decrease) in net unrealized gains in OCI, net of deferred income tax |  | $ | 38,453 |  |  | (2,873 | ) |  | (35,398 | )
Schedule of Held-to-maturity Securities |
December 31, 2017 |  |  |  | Net |  |  |  |  |  |  |  |
|  |  |  | Unrealized |  |  |  | Unrecognized |  | Unrecognized |  |
|  | Amortized |  | Gains |  | Carrying |  | Holding |  | Holding |  | Fair
($ in thousands) |  | Cost |  | (Losses) |  | Value |  | Gains |  | Losses |  | Value
Obligations of state and political subdivisions |  | $ | 25,154 |  |  | 84 |  |  | 25,238 |  |  | 1,023 |  |  | — |  |  | 26,261 |
Corporate securities |  | 16,996 |  |  | (105 | ) |  | 16,891 |  |  | 1,003 |  |  | (55 | ) |  | 17,839 |
Total HTM fixed income securities |  | $ | 42,150 |  |  | (21 | ) |  | 42,129 |  |  | 2,026 |  |  | (55 | ) |  | 44,100 |
December 31, 2016 |  |  |  | Net |  |  |  |  |  |  |  |
|  |  |  | Unrealized |  |  |  | Unrecognized |  | Unrecognized |  |
|  | Amortized |  | Gains |  | Carrying |  | Holding |  | Holding |  | Fair
($ in thousands) |  | Cost |  | (Losses) |  | Value |  | Gains |  | Losses |  | Value
Obligations of state and political subdivisions |  | 77,466 |  |  | 317 |  |  | 77,783 |  |  | 2,133 |  |  | — |  |  | 79,916 |
Corporate securities |  | 22,711 |  |  | (143 | ) |  | 22,568 |  |  | 1,665 |  |  | (158 | ) |  | 24,075 |
CMBS |  | 1,220 |  |  | (15 | ) |  | 1,205 |  |  | 15 |  |  | — |  |  | 1,220 |
Total HTM fixed income securities |  | $ | 101,397 |  |  | 159 |  |  | 101,556 |  |  | 3,813 |  |  | (158 | ) |  | 105,211 |
Schedule of Available For Sale Securities |
December 31, 2017 |  |  |  |  |  |  |  |
|  | Cost/ |  |  |  |  |  |
|  | Amortized |  | Unrealized |  | Unrealized |  | Fair
($ in thousands) |  | Cost |  | Gains |  | Losses |  | Value
AFS fixed income securities: |  |  |  |  |  |  |  |
U.S. government and government agencies |  | $ | 49,326 |  |  | 647 |  |  | (233 | ) |  | 49,740 |
Foreign government |  | 18,040 |  |  | 526 |  |  | (11 | ) |  | 18,555 |
Obligations of states and political subdivisions |  | 1,539,307 |  |  | 44,245 |  |  | (582 | ) |  | 1,582,970 |
Corporate securities |  | 1,588,339 |  |  | 30,891 |  |  | (1,762 | ) |  | 1,617,468 |
CLO and other ABS |  | 789,152 |  |  | 6,508 |  |  | (202 | ) |  | 795,458 |
CMBS |  | 382,727 |  |  | 1,563 |  |  | (841 | ) |  | 383,449 |
RMBS |  | 709,825 |  |  | 6,487 |  |  | (1,430 | ) |  | 714,882 |
Total AFS fixed income securities |  | 5,076,716 |  |  | 90,867 |  |  | (5,061 | ) |  | 5,162,522 |
AFS equity securities: |  |  |  |  |  |  |  |
Common stock |  | 129,696 |  |  | 38,287 |  |  | (226 | ) |  | 167,757 |
Preferred stock |  | 14,115 |  |  | 904 |  |  | (71 | ) |  | 14,948 |
Total AFS equity securities |  | 143,811 |  |  | 39,191 |  |  | (297 | ) |  | 182,705 |
Total AFS securities |  | $ | 5,220,527 |  |  | 130,058 |  |  | (5,358 | ) |  | 5,345,227 |
December 31, 2016 |  |  |  |  |  |  |  |
|  | Cost/ |  |  |  |  |  |
|  | Amortized |  | Unrealized |  | Unrealized |  | Fair
($ in thousands) |  | Cost |  | Gains |  | Losses |  | Value
AFS fixed income securities: |  |  |  |  |  |  |  |
U.S. government and government agencies |  | $ | 75,139 |  |  | 2,230 |  |  | (36 | ) |  | 77,333 |
Foreign government |  | 26,559 |  |  | 322 |  |  | (16 | ) |  | 26,865 |
Obligations of states and political subdivisions |  | 1,366,287 |  |  | 18,610 |  |  | (5,304 | ) |  | 1,379,593 |
Corporate securities |  | 1,976,556 |  |  | 27,057 |  |  | (5,860 | ) |  | 1,997,753 |
CLO and other ABS |  | 527,876 |  |  | 1,439 |  |  | (355 | ) |  | 528,960 |
CMBS |  | 256,356 |  |  | 1,514 |  |  | (1,028 | ) |  | 256,842 |
RMBS |  | 524,986 |  |  | 3,006 |  |  | (2,798 | ) |  | 525,194 |
Total AFS fixed income securities |  | 4,753,759 |  |  | 54,178 |  |  | (15,397 | ) |  | 4,792,540 |
AFS equity securities: |  |  |  |  |  |  |  |
Common stock |  | 104,663 |  |  | 26,250 |  |  | (305 | ) |  | 130,608 |
Preferred stock |  | 16,226 |  |  | 274 |  |  | (355 | ) |  | 16,145 |
Total AFS equity securities |  | 120,889 |  |  | 26,524 |  |  | (660 | ) |  | 146,753 |
Total AFS securities |  | $ | 4,874,648 |  |  | 80,702 |  |  | (16,057 | ) |  | 4,939,293 |
Schedule of Fair Value and Gross Pre-Tax Net Unrealized/Unrecognized Loss of Securities by Length of Time |
December 31, 2017 |  | Less than 12 months |  | 12 months or longer |  | Total
($ in thousands) |  | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses
AFS fixed income securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. government and government agencies |  | $ | 23,516 |  |  | (233 | ) |  | 250 |  |  | — |  |  | 23,766 |  |  | (233 | )
Foreign government |  | 1,481 |  |  | (11 | ) |  | — |  |  | — |  |  | 1,481 |  |  | (11 | )
Obligations of states and political subdivisions |  | 107,514 |  |  | (422 | ) |  | 14,139 |  |  | (160 | ) |  | 121,653 |  |  | (582 | )
Corporate securities |  | 238,326 |  |  | (1,744 | ) |  | 3,228 |  |  | (18 | ) |  | 241,554 |  |  | (1,762 | )
CLO and other ABS |  | 74,977 |  |  | (196 | ) |  | 1,655 |  |  | (6 | ) |  | 76,632 |  |  | (202 | )
CMBS |  | 154,267 |  |  | (773 | ) |  | 5,214 |  |  | (68 | ) |  | 159,481 |  |  | (841 | )
RMBS |  | 269,485 |  |  | (1,285 | ) |  | 11,200 |  |  | (145 | ) |  | 280,685 |  |  | (1,430 | )
Total AFS fixed income securities |  | 869,566 |  |  | (4,664 | ) |  | 35,686 |  |  | (397 | ) |  | 905,252 |  |  | (5,061 | )
AFS equity securities: |  |  |  |  |  |  |  |  |  |  |  |
Common stock |  | 4,727 |  |  | (226 | ) |  | — |  |  | — |  |  | 4,727 |  |  | (226 | )
Preferred stock |  | 3,833 |  |  | (71 | ) |  | — |  |  | — |  |  | 3,833 |  |  | (71 | )
Total AFS equity securities |  | 8,560 |  |  | (297 | ) |  | — |  |  | — |  |  | 8,560 |  |  | (297 | )
Total AFS securities |  | $ | 878,126 |  |  | (4,961 | ) |  | 35,686 |  |  | (397 | ) |  | 913,812 |  |  | (5,358 | )
December 31, 2016 |  | Less than 12 months |  | 12 months or longer |  | Total
($ in thousands) |  | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses |  | Fair Value |  | Unrealized Losses
AFS fixed income securities: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. government and government agencies |  | $ | 6,419 |  |  | (36 | ) |  | — |  |  | — |  |  | $ | 6,419 |  |  | $ | (36 | )
Foreign government |  | 13,075 |  |  | (16 | ) |  | — |  |  | — |  |  | 13,075 |  |  | (16 | )
Obligations of states and political subdivisions |  | 306,509 |  |  | (5,304 | ) |  | — |  |  | — |  |  | 306,509 |  |  | (5,304 | )
Corporate securities |  | 462,902 |  |  | (5,771 | ) |  | 4,913 |  |  | (89 | ) |  | 467,815 |  |  | (5,860 | )
CLO and other ABS |  | 189,795 |  |  | (354 | ) |  | 319 |  |  | (1 | ) |  | 190,114 |  |  | (355 | )
CMBS |  | 82,492 |  |  | (1,021 | ) |  | 1,645 |  |  | (7 | ) |  | 84,137 |  |  | (1,028 | )
RMBS |  | 279,480 |  |  | (2,489 | ) |  | 8,749 |  |  | (309 | ) |  | 288,229 |  |  | (2,798 | )
Total AFS fixed income securities |  | 1,340,672 |  |  | (14,991 | ) |  | 15,626 |  |  | (406 | ) |  | 1,356,298 |  |  | (15,397 | )
AFS equity securities: |  |  |  |  |  |  |  |  |  |  |  |
Common stock |  | 11,271 |  |  | (305 | ) |  | — |  |  | — |  |  | 11,271 |  |  | (305 | )
Preferred stock |  | 6,168 |  |  | (355 | ) |  | — |  |  | — |  |  | 6,168 |  |  | (355 | )
Total AFS equity securities |  | 17,439 |  |  | (660 | ) |  | — |  |  | — |  |  | 17,439 |  |  | (660 | )
Total AFS securities |  | $ | 1,358,111 |  |  | (15,651 | ) |  | 15,626 |  |  | (406 | ) |  | $ | 1,373,737 |  |  | $ | (16,057 | )
Investments Classified by Contractual Maturity Date [Table Text Block] |
|  | AFS |  | HTM
($ in thousands) |  | Fair Value |  | Carrying Value |  | Fair Value
Due in one year or less |  | $ | 315,857 |  |  | 10,997 |  |  | 11,168 |
Due after one year through five years |  | 2,099,529 |  |  | 23,035 |  |  | 24,235 |
Due after five years through 10 years |  | 2,510,294 |  |  | 8,097 |  |  | 8,697 |
Due after 10 years |  | 236,842 |  |  | — |  |  | — |
Total fixed income securities |  | $ | 5,162,522 |  |  | 42,129 |  |  | 44,100 |
Schedule of Other Investment Portfolio by Strategy and the Remaining Commitment Amount Associated With Each Strategy |
Other Investments |  | December 31, 2017 |  | December 31, 2016
($ in thousands) |  | Carrying Value |  | Remaining Commitment |  | Maximum Exposure to Loss 1 |  | Carrying Value |  | Remaining Commitment |  | Maximum Exposure to Loss 1
Alternative Investments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Private equity |  | $ | 52,251 |  |  | 99,026 |  |  | 151,277 |  |  | 41,135 |  |  | 76,774 |  |  | 117,909 |
Private credit |  | 37,743 |  |  | 94,959 |  |  | 132,702 |  |  | 28,193 |  |  | 40,613 |  |  | 68,806 |
Real assets |  | 25,379 |  |  | 27,014 |  |  | 52,393 |  |  | 14,486 |  |  | 22,899 |  |  | 37,385 |
Total alternative investments |  | 115,373 |  |  | 220,999 |  |  | 336,372 |  |  | 83,814 |  |  | 140,286 |  |  | 224,100 |
Other securities 2 |  | 16,895 |  |  | — |  |  | 16,895 |  |  | 18,583 |  |  | 3,400 |  |  | 21,983 |
Total other investments |  | $ | 132,268 |  |  | 220,999 |  |  | 353,267 |  |  | 102,397 |  |  | 143,686 |  |  | 246,083 |
Schedule of Aggregated Summarized Balance Sheet Financial Information For Partnerhips In Our Alternative Investment Portfolio |
Balance Sheet Information |  |  |  |
September 30, |  |  |  |
($ in millions) |  | 2017 |  | 2016
Investments |  | $ | 21,046 |  |  | 11,244 |
Total assets |  | 22,357 |  |  | 12,075 |
Total liabilities |  | 4,767 |  |  | 1,802 |
Total partners’ capital |  | 17,590 |  |  | 10,273 |
Schedule of Aggregated Summarized Income Statement Financial Information For Partnerhips In Our Alternative Investment Portfolio |
Income Statement Information |  |  |  |  |  |
12 months ended September 30, |  |  |  |  |  |
($ in millions) |  | 2017 |  | 2016 |  | 2015
Net investment (loss) income |  | $ | (143 | ) |  | (44 | ) |  | 129 |
Realized gains |  | 325 |  |  | 1,374 |  |  | 1,187 |
Net change in unrealized appreciation (depreciation) |  | 2,894 |  |  | (719 | ) |  | (1,364 | )
Net income |  | $ | 3,076 |  |  | 611 |  |  | (48 | )
Insurance Subsidiaries' alternative investments income (loss) |  | 12.7 |  |  | 3.1 |  |  | (1.9 | )
Schedule Of Securities Pledged As Collateral | The following table summarizes the market value of these securities at December 31, 2017:
($ in millions) |  | FHLBI Collateral |  | FHLBNY Collateral |  | State and Regulatory Deposits |  | Total
U.S. government and government agencies |  | $ | 3.0 |  |  | — |  |  | 22.6 |  |  | 25.6 |
Obligations of states and political subdivisions |  | — |  |  | — |  |  | 3.1 |  |  | 3.1 |
CMBS |  | 6.2 |  |  | 14.1 |  |  | — |  |  | 20.3 |
RMBS |  | 56.3 |  |  | 59.6 |  |  | — |  |  | 115.9 |
Total pledged as collateral |  | $ | 65.5 |  |  | 73.7 |  |  | 25.7 |  |  | 164.9 |
Schedule of Components of Pre-Tax Net Investment Income Earned |
($ in thousands) |  | 2017 |  | 2016 |  | 2015
Fixed income securities |  | $ | 153,230 |  |  | 129,306 |  |  | 123,230 |
Equity securities |  | 6,442 |  |  | 7,368 |  |  | 9,161 |
Short-term investments |  | 1,526 |  |  | 686 |  |  | 112 |
Other investments |  | 12,871 |  |  | 2,940 |  |  | (1,890 | )
Investment expenses |  | (12,187 | ) |  | (9,546 | ) |  | (9,297 | )
Net investment income earned |  | $ | 161,882 |  |  | 130,754 |  |  | 121,316 |
Schedule of OTTI by Asset Type |
2017 |  |  |  |  |  | Recognized in Earnings
($ in thousands) |  | Gross |  | Included in OCI |
AFS fixed income securities: |  |  |  |  |  |
U.S. government and government agencies |  | $ | 36 |  |  | — |  |  | 36 |
Obligations of states and political subdivisions |  | 612 |  |  | — |  |  | 612 |
Corporate securities |  | 587 |  |  | — |  |  | 587 |
CLO and other ABS |  | 96 |  |  | — |  |  | 96 |
CMBS |  | 670 |  |  | — |  |  | 670 |
RMBS |  | 1,183 |  |  | (36 | ) |  | 1,219 |
Total AFS fixed income securities |  | 3,184 |  |  | (36 | ) |  | 3,220 |
AFS equity securities: |  |  |  |  |  |
Common stock |  | 1,435 |  |  | — |  |  | 1,435 |
Total AFS equity securities |  | 1,435 |  |  | — |  |  | 1,435 |
Other investments |  | $ | 190 |  |  | — |  |  | 190 |
Total OTTI losses |  | $ | 4,809 |  |  | (36 | ) |  | 4,845 |
2016 |  |  |  |  |  | Recognized in Earnings
($ in thousands) |  | Gross |  | Included in OCI |
AFS fixed income securities: |  |  |  |  |  |
Obligations of states and political subdivisons |  | $ | 2,797 |  |  | — |  |  | 2,797 |
Corporate securities |  | 1,880 |  |  | — |  |  | 1,880 |
CLO and other ABS |  | 19 |  |  | — |  |  | 19 |
CMBS |  | 220 |  |  | — |  |  | 220 |
RMBS |  | 275 |  |  | 10 |  |  | 265 |
Total AFS fixed income securities |  | 5,191 |  |  | 10 |  |  | 5,181 |
AFS equity securities: |  |  |  |  |  |
Common stock |  | 3,316 |  |  | — |  |  | 3,316 |
Preferred stock |  | 2 |  |  | — |  |  | 2 |
Total AFS equity securities |  | 3,318 |  |  | — |  |  | 3,318 |
Total OTTI losses |  | $ | 8,509 |  |  | 10 |  |  | 8,499 |
2015 |  |  |  |  |  | Recognized in Earnings
($ in thousands) |  | Gross |  | Included in OCI |
AFS fixed income securities: |  |  |  |  |  |  |  |  |
Corporate securities |  | $ | 2,188 |  |  | — |  |  | 2,188 |
RMBS |  | 1 |  |  | — |  |  | 1 |
Total AFS fixed income securities |  | 2,189 |  |  | — |  |  | 2,189 |
AFS equity securities: |  |  |  |  |  |
Common stock |  | 15,996 |  |  | — |  |  | 15,996 |
Preferred stock |  | 181 |  |  | — |  |  | 181 |
Total AFS equity securities |  | 16,177 |  |  | — |  |  | 16,177 |
Total OTTI losses |  | $ | 18,366 |  |  | — |  |  | 18,366 |
Schedule of Components of Net Realized Gains Excluding OTTI Charges |
($ in thousands) |  | 2017 |  | 2016 |  | 2015
HTM fixed income securities |  |  |  |  |  |  |  |  |
Gains |  | $ | 44 |  |  | 3 |  |  | 5 |
Losses |  | (1 | ) |  | (1 | ) |  | (1 | )
AFS fixed income securities |  |  |  |  |  |  |  |  |
Gains |  | 10,193 |  |  | 7,741 |  |  | 4,515 |
Losses |  | (3,292 | ) |  | (11,411 | ) |  | (312 | )
AFS equity securities |  |  |  |  |  |  |  |  |
Gains |  | 5,829 |  |  | 8,108 |  |  | 29,168 |
Losses |  | (1,200 | ) |  | (864 | ) |  | (1,347 | )
Short-term investments |  |  |  |  |  |
Gains |  | 2 |  |  | — |  |  | — |
Losses |  | (6 | ) |  | (13 | ) |  | — |
Other investments |  |  |  |  |  |  |  |  |
Gains |  | 494 |  |  | 3 |  |  | 162 |
Losses |  | (859 | ) |  | (4 | ) |  | (653 | )
Total net realized investment gains |  | $ | 11,204 |  |  | 3,562 |  |  | 31,537 |

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