Investments (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($)
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period	$ 5,196,784,000	$ 4,946,278,000
Total at end of period	5,151,987,000	4,983,315,000
Assets held by insurance regulatory requirements	13,500,000	13,600,000
Assets held in trust for the benefit of contractual counterparties	26,300,000	7,700,000
Maturities, Amortized Cost [Abstract]
Due in one year or less	484,485,000
Due after one year through five years	1,652,638,000
Due after five years through ten years	1,011,237,000
Due after ten years	1,160,294,000
Total debt securities with single maturity date	4,308,654,000
Total at end of period	5,196,784,000	4,946,278,000
Maturities, Fair Value [Abstract]
Due in one year or less	482,919,000
Due after one year through five years	1,632,494,000
Due after five years through ten years	996,335,000
Due after ten years	1,176,290,000
Total debt securities with single maturity date	4,288,038,000
Total at end of period	5,151,987,000	4,983,315,000
Other than temporary impairments recorded in other comprehensive (loss) income	0	0
Equity Securities, FV-NI, Gain (Loss) [Abstract]
Equity securities, cost	3,993,000
Equity securities, at fair value	$ 3,932,000
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Equity securities, cost		7,223,000
Equity securities, at fair value		7,246,000
Collateral, principal balance of FHLB (as a percent)	102.00%
Collateral, principal balance of FHLB	$ 168,900,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	2,026,193,000	1,667,170,000
12 months or greater	1,554,704,000	752,693,000
Total investment portfolio	3,580,897,000	2,419,863,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(28,520,000)	(9,357,000)
12 months or greater	(49,384,000)	(22,206,000)
Total investment portfolio	$ (77,904,000)	$ (31,563,000)
Number of securities in unrealized loss position | security	721	586
For securities in an unrealized loss position, percentage of fair value to amortized cost (in hundredths)	98.00%
For securities in an unrealized loss position, percentage backed by the U.S. Government (in hundredths)	8.00%
Total fixed income securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period	$ 5,196,784,000	$ 4,946,278,000
Gross Unrealized Gains	33,107,000	68,584,000
Gross Unrealized Losses	(77,904,000)	(31,547,000)
Total at end of period	5,151,987,000	4,983,315,000
Maturities, Amortized Cost [Abstract]
Total at end of period	5,196,784,000	4,946,278,000
Maturities, Fair Value [Abstract]
Total at end of period	5,151,987,000	4,983,315,000
Proceeds from sale of fixed income securities available for sale	365,600,000	246,900,000	$ 728,000,000
Gross realized gains	700,000	1,600,000	11,900,000
Gross realized losses	3,800,000	1,400,000	3,000,000
Recognized other than temporary impairments	1,800,000	0	$ 0
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	167,655,000	179,850,000
Gross Unrealized Gains	597,000	274,000
Gross Unrealized Losses	(1,076,000)	(1,278,000)
Total at end of period	167,176,000	178,846,000
Maturities, Amortized Cost [Abstract]
Total at end of period	167,655,000	179,850,000
Maturities, Fair Value [Abstract]
Total at end of period	167,176,000	178,846,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	23,710,000	144,042,000
12 months or greater	69,146,000	31,196,000
Total investment portfolio	92,856,000	175,238,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(15,000)	(796,000)
12 months or greater	(1,061,000)	(482,000)
Total investment portfolio	(1,076,000)	(1,278,000)
Obligations of U.S. states and political subdivisions
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period	1,701,826,000	2,105,063,000
Gross Unrealized Gains	29,259,000	56,210,000
Gross Unrealized Losses	(10,985,000)	(8,749,000)
Total at end of period	1,720,100,000	2,152,524,000
Maturities, Amortized Cost [Abstract]
Total at end of period	1,701,826,000	2,105,063,000
Maturities, Fair Value [Abstract]
Total at end of period	1,720,100,000	2,152,524,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	316,655,000	505,311,000
12 months or greater	358,086,000	211,684,000
Total investment portfolio	674,741,000	716,995,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(3,875,000)	(3,624,000)
12 months or greater	(7,110,000)	(5,125,000)
Total investment portfolio	(10,985,000)	(8,749,000)
Corporate debt securities
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period	2,439,173,000	2,065,475,000
Gross Unrealized Gains	2,103,000	10,532,000
Gross Unrealized Losses	(40,514,000)	(9,169,000)
Total at end of period	2,400,762,000	2,066,838,000
Maturities, Amortized Cost [Abstract]
Total at end of period	2,439,173,000	2,065,475,000
Maturities, Fair Value [Abstract]
Total at end of period	2,400,762,000	2,066,838,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	1,272,279,000	932,350,000
12 months or greater	785,627,000	200,716,000
Total investment portfolio	2,057,906,000	1,133,066,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(18,130,000)	(4,288,000)
12 months or greater	(22,384,000)	(4,881,000)
Total investment portfolio	(40,514,000)	(9,169,000)
ABS
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period	111,953,000	4,925,000
Gross Unrealized Gains	226,000	0
Gross Unrealized Losses	(146,000)	(2,000)
Total at end of period	112,033,000	4,923,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	111,953,000
Total at end of period	111,953,000	4,925,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	112,033,000
Total at end of period	112,033,000	4,923,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	51,324,000	4,923,000
12 months or greater	0	0
Total investment portfolio	51,324,000	4,923,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(146,000)	(2,000)
12 months or greater	0	0
Total investment portfolio	(146,000)	(2,000)
RMBS
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period	189,238,000	189,153,000
Gross Unrealized Gains	32,000	60,000
Gross Unrealized Losses	(10,309,000)	(7,364,000)
Total at end of period	178,961,000	181,849,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	189,238,000
Total at end of period	189,238,000	189,153,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	178,961,000
Total at end of period	178,961,000	181,849,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	24,000	14,979,000
12 months or greater	178,573,000	166,329,000
Total investment portfolio	178,597,000	181,308,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	0	(280,000)
12 months or greater	(10,309,000)	(7,084,000)
Total investment portfolio	(10,309,000)	(7,364,000)
CMBS
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period	276,352,000	301,014,000
Gross Unrealized Gains	888,000	1,204,000
Gross Unrealized Losses	(9,580,000)	(4,906,000)
Total at end of period	267,660,000	297,312,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	276,352,000
Total at end of period	276,352,000	301,014,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	267,660,000
Total at end of period	267,660,000	297,312,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	65,704,000	51,096,000
12 months or greater	163,272,000	138,769,000
Total investment portfolio	228,976,000	189,865,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(1,060,000)	(358,000)
12 months or greater	(8,520,000)	(4,548,000)
Total investment portfolio	(9,580,000)	(4,906,000)
CLOs
Available-for-sale Debt Securities, Amortized Cost Basis [Abstract]
Total at end of period	310,587,000	100,798,000
Gross Unrealized Gains	2,000	304,000
Gross Unrealized Losses	(5,294,000)	(79,000)
Total at end of period	305,295,000	101,023,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	310,587,000
Total at end of period	310,587,000	100,798,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	305,295,000
Total at end of period	305,295,000	101,023,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months	296,497,000	14,243,000
12 months or greater	0	3,568,000
Total investment portfolio	296,497,000	17,811,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	(5,294,000)	(7,000)
12 months or greater	0	(72,000)
Total investment portfolio	(5,294,000)	(79,000)
Equity securities
Equity Securities, FV-NI, Gain (Loss) [Abstract]
Equity securities, cost	3,993,000
Gross gains	11,000
Gross losses	(72,000)
Equity securities, at fair value	3,932,000
Proceeds from sale of equity securities	4,900,000
Gross realized gains on equity securities	3,700,000
Recognized net losses on equity securities still held	$ 84,000
Available-for-sale Equity Securities, Amortized Cost Basis [Abstract]
Equity securities, cost		7,223,000
Gross gains		39,000
Gross losses		(16,000)
Equity securities, at fair value		7,246,000
Continuous Unrealized Loss Portion, Fair Value [Abstract]
Less than 12 months		226,000
12 months or greater		431,000
Total investment portfolio		657,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months		(2,000)
12 months or greater		(14,000)
Total investment portfolio		$ (16,000)