Investments (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 29, 2016	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		$ 4,946,278	$ 4,717,211
Total at end of period		4,983,315	4,685,222
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Fair Value		7,246	7,128
Total Investment Portfolio [Abstract]
Amortized Cost		4,953,501	4,724,355
Gross Unrealized Gains		68,623	28,628
Gross Unrealized Losses		(31,563)	(60,633)
Fair Value		4,990,561	4,692,350
Other than temporary impairments recorded in other comprehensive (loss) income		$ 0	0
Collateral, principal balance of FHLB (as a percent)	102.00%	102.00%
Collateral, principal balance of FHLB		$ 166,900
Assets held by insurance regulatory requirements		13,600	13,600
Maturities, Amortized Cost [Abstract]
Due in one year or less		541,755
Due after one year through five years		1,547,712
Due after five years through ten years		925,751
Due after ten years		1,335,170
Total debt securities with single maturity date		4,350,388
Total at end of period		4,946,278	4,717,211
Maturities, Fair Value [Abstract]
Due in one year or less		541,695
Due after one year through five years		1,544,943
Due after five years through ten years		929,883
Due after ten years		1,381,687
Total debt securities with single maturity date		4,398,208
Total at end of period		4,983,315	4,685,222
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		1,667,170	2,370,345
12 months or greater		752,693	294,916
Total investment portfolio		2,419,863	2,665,261
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(9,357)	(50,915)
12 months or greater		(22,206)	(9,718)
Total investment portfolio		$ (31,563)	$ (60,633)
Number of securities in unrealized loss position | security		586	607
For securities in an unrealized loss position, percentage of fair value to amortized cost (in hundredths)		99.00%
For securities in an unrealized loss position, percentage backed by the U.S. Government (in hundredths)		27.00%
Total debt securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		$ 4,946,278	$ 4,717,211
Gross Unrealized Gains		68,584	28,620
Gross Unrealized Losses		(31,547)	(60,609)
Total at end of period		4,983,315	4,685,222
Total Investment Portfolio [Abstract]
Fair Value		4,983,315	4,685,222
Maturities, Amortized Cost [Abstract]
Total at end of period		4,946,278	4,717,211
Maturities, Fair Value [Abstract]
Total at end of period		4,983,315	4,685,222
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		179,850	73,847
Gross Unrealized Gains		274	407
Gross Unrealized Losses		(1,278)	(724)
Total at end of period		178,846	73,530
Total Investment Portfolio [Abstract]
Fair Value		178,846	73,530
Maturities, Amortized Cost [Abstract]
Total at end of period		179,850	73,847
Maturities, Fair Value [Abstract]
Total at end of period		178,846	73,530
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		144,042	48,642
12 months or greater		31,196	0
Total investment portfolio		175,238	48,642
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(796)	(724)
12 months or greater		(482)	0
Total investment portfolio		(1,278)	(724)
Obligations of U.S. states and political subdivisions
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		2,105,063	2,147,458
Gross Unrealized Gains		56,210	20,983
Gross Unrealized Losses		(8,749)	(25,425)
Total at end of period		2,152,524	2,143,016
Total Investment Portfolio [Abstract]
Fair Value		2,152,524	2,143,016
Maturities, Amortized Cost [Abstract]
Total at end of period		2,105,063	2,147,458
Maturities, Fair Value [Abstract]
Total at end of period		2,152,524	2,143,016
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		505,311	1,136,676
12 months or greater		211,684	13,681
Total investment portfolio		716,995	1,150,357
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(3,624)	(24,918)
12 months or greater		(5,125)	(507)
Total investment portfolio		(8,749)	(25,425)
Corporate debt securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		2,065,475	1,756,461
Gross Unrealized Gains		10,532	6,059
Gross Unrealized Losses		(9,169)	(18,610)
Total at end of period		2,066,838	1,743,910
Total Investment Portfolio [Abstract]
Fair Value		2,066,838	1,743,910
Maturities, Amortized Cost [Abstract]
Total at end of period		2,065,475	1,756,461
Maturities, Fair Value [Abstract]
Total at end of period		2,066,838	1,743,910
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		932,350	915,777
12 months or greater		200,716	35,769
Total investment portfolio		1,133,066	951,546
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(4,288)	(16,771)
12 months or greater		(4,881)	(1,839)
Total investment portfolio		(9,169)	(18,610)
ABS
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		4,925	59,519
Gross Unrealized Gains		0	74
Gross Unrealized Losses		(2)	(28)
Total at end of period		4,923	59,565
Total Investment Portfolio [Abstract]
Fair Value		4,923	59,565
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date		4,925
Total at end of period		4,925	59,519
Maturities, Fair Value [Abstract]
Debt securities without single maturity date		4,923
Total at end of period		4,923	59,565
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		4,923	3,366
12 months or greater		0	656
Total investment portfolio		4,923	4,022
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(2)	(28)
12 months or greater		0	0
Total investment portfolio		(2)	(28)
RMBS
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		189,153	231,733
Gross Unrealized Gains		60	102
Gross Unrealized Losses		(7,364)	(7,626)
Total at end of period		181,849	224,209
Total Investment Portfolio [Abstract]
Fair Value		181,849	224,209
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date		189,153
Total at end of period		189,153	231,733
Maturities, Fair Value [Abstract]
Debt securities without single maturity date		181,849
Total at end of period		181,849	224,209
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		14,979	46,493
12 months or greater		166,329	171,326
Total investment portfolio		181,308	217,819
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(280)	(857)
12 months or greater		(7,084)	(6,769)
Total investment portfolio		(7,364)	(7,626)
CMBS
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		301,014	327,042
Gross Unrealized Gains		1,204	769
Gross Unrealized Losses		(4,906)	(7,994)
Total at end of period		297,312	319,817
Total Investment Portfolio [Abstract]
Fair Value		297,312	319,817
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date		301,014
Total at end of period		301,014	327,042
Maturities, Fair Value [Abstract]
Debt securities without single maturity date		297,312
Total at end of period		297,312	319,817
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		51,096	205,545
12 months or greater		138,769	38,587
Total investment portfolio		189,865	244,132
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(358)	(7,529)
12 months or greater		(4,548)	(465)
Total investment portfolio		(4,906)	(7,994)
CLOs
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period		100,798	121,151
Gross Unrealized Gains		304	226
Gross Unrealized Losses		(79)	(202)
Total at end of period		101,023	121,175
Total Investment Portfolio [Abstract]
Fair Value		101,023	121,175
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date		100,798
Total at end of period		100,798	121,151
Maturities, Fair Value [Abstract]
Debt securities without single maturity date		101,023
Total at end of period		101,023	121,175
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		14,243	13,278
12 months or greater		3,568	34,760
Total investment portfolio		17,811	48,038
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(7)	(73)
12 months or greater		(72)	(129)
Total investment portfolio		(79)	(202)
Equity securities
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Amortized Cost		7,223	7,144
Gross Unrealized Gains		39	8
Gross Unrealized Losses		(16)	(24)
Fair Value		7,246	7,128
Total Investment Portfolio [Abstract]
Fair Value		7,246	7,128
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		226	568
12 months or greater		431	137
Total investment portfolio		657	705
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(2)	(15)
12 months or greater		(14)	(9)
Total investment portfolio		$ (16)	$ (24)