Investments (Details)	12 Months Ended
	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale [Line Items]
Total at end of period	$ 5,562,550,000	$ 5,196,784,000
Total at end of period	5,737,892,000	5,151,987,000
Other than temporary impairments recorded in other comprehensive (loss) income	0	0
Assets held by insurance regulatory requirements	13,900,000	13,500,000
Assets held in trust for the benefit of contractual counterparties	$ 89,900,000	26,300,000
Collateral, principal balance of FHLB (as a percent)	102.00%
Collateral, principal balance of FHLB	$ 165,700,000
Maturities, Amortized Cost [Abstract]
Due in one year or less	425,739,000
Due after one year through five years	1,911,433,000
Due after five years through ten years	1,031,056,000
Due after ten years	1,094,252,000
Total debt securities with single maturity date	4,462,480,000
Total at end of period	5,562,550,000	5,196,784,000
Maturities, Fair Value [Abstract]
Due in one year or less	427,616,000
Due after one year through five years	1,952,278,000
Due after five years through ten years	1,088,012,000
Due after ten years	1,167,284,000
Total debt securities with single maturity date	4,635,190,000
Total at end of period	5,737,892,000	5,151,987,000
Equity Securities, FV-NI, Gain (Loss) [Abstract]
Equity securities, cost	17,188,000	3,993,000
Equity securities, at fair value (cost, 2019 - $17,188; 2018 - $3,993)	17,328,000	3,932,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	624,334,000	2,026,193,000
12 months or greater	379,411,000	1,554,704,000
Total	1,003,745,000	3,580,897,000
Less than 12 months	(5,426,000)	(28,520,000)
12 months or greater	(4,476,000)	(49,384,000)
Total	$ (9,902,000)	$ (77,904,000)
Number of securities in unrealized loss position | security	217	721
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)	28.00%
Total fixed income securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	$ 5,562,550,000	$ 5,196,784,000
Gross Unrealized Gains	185,244,000	33,107,000
Gross Unrealized Losses	(9,902,000)	(77,904,000)
Total at end of period	5,737,892,000	5,151,987,000
Maturities, Amortized Cost [Abstract]
Total at end of period	5,562,550,000	5,196,784,000
Maturities, Fair Value [Abstract]
Total at end of period	5,737,892,000	5,151,987,000
Proceeds from sale of fixed income securities available for sale	228,100,000	365,600,000	$ 246,900,000
Gross realized gains	7,100,000	700,000	1,600,000
Gross realized losses	3,500,000	3,800,000	1,400,000
Recognized other than temporary impairments	100,000	1,800,000	$ 0
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Debt Securities, Available-for-sale [Line Items]
Total at end of period	195,176,000	167,655,000
Gross Unrealized Gains	1,237,000	597,000
Gross Unrealized Losses	(210,000)	(1,076,000)
Total at end of period	196,203,000	167,176,000
Maturities, Amortized Cost [Abstract]
Total at end of period	195,176,000	167,655,000
Maturities, Fair Value [Abstract]
Total at end of period	196,203,000	167,176,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	57,301,000	23,710,000
12 months or greater	5,806,000	69,146,000
Total	63,107,000	92,856,000
Less than 12 months	(200,000)	(15,000)
12 months or greater	(10,000)	(1,061,000)
Total	(210,000)	(1,076,000)
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Total at end of period	1,555,394,000	1,701,826,000
Gross Unrealized Gains	99,328,000	29,259,000
Gross Unrealized Losses	(857,000)	(10,985,000)
Total at end of period	1,653,865,000	1,720,100,000
Maturities, Amortized Cost [Abstract]
Total at end of period	1,555,394,000	1,701,826,000
Maturities, Fair Value [Abstract]
Total at end of period	1,653,865,000	1,720,100,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	74,859,000	316,655,000
12 months or greater	6,957,000	358,086,000
Total	81,816,000	674,741,000
Less than 12 months	(847,000)	(3,875,000)
12 months or greater	(10,000)	(7,110,000)
Total	(857,000)	(10,985,000)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	2,711,910,000	2,439,173,000
Gross Unrealized Gains	76,220,000	2,103,000
Gross Unrealized Losses	(3,008,000)	(40,514,000)
Total at end of period	2,785,122,000	2,400,762,000
Maturities, Amortized Cost [Abstract]
Total at end of period	2,711,910,000	2,439,173,000
Maturities, Fair Value [Abstract]
Total at end of period	2,785,122,000	2,400,762,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	221,357,000	1,272,279,000
12 months or greater	43,505,000	785,627,000
Total	264,862,000	2,057,906,000
Less than 12 months	(2,847,000)	(18,130,000)
12 months or greater	(161,000)	(22,384,000)
Total	(3,008,000)	(40,514,000)
ABS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	227,376,000	111,953,000
Gross Unrealized Gains	2,466,000	226,000
Gross Unrealized Losses	(178,000)	(146,000)
Total at end of period	229,664,000	112,033,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	227,376,000
Total at end of period	227,376,000	111,953,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	229,664,000
Total at end of period	229,664,000	112,033,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	21,542,000	51,324,000
12 months or greater	3,851,000	0
Total	25,393,000	51,324,000
Less than 12 months	(118,000)	(146,000)
12 months or greater	(60,000)	0
Total	(178,000)	(146,000)
RMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	271,384,000	189,238,000
Gross Unrealized Gains	429,000	32,000
Gross Unrealized Losses	(3,227,000)	(10,309,000)
Total at end of period	268,586,000	178,961,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	271,384,000
Total at end of period	271,384,000	189,238,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	268,586,000
Total at end of period	268,586,000	178,961,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	105,443,000	24,000
12 months or greater	110,452,000	178,573,000
Total	215,895,000	178,597,000
Less than 12 months	(461,000)	0
12 months or greater	(2,766,000)	(10,309,000)
Total	(3,227,000)	(10,309,000)
CMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	274,234,000	276,352,000
Gross Unrealized Gains	5,531,000	888,000
Gross Unrealized Losses	(779,000)	(9,580,000)
Total at end of period	278,986,000	267,660,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	274,234,000
Total at end of period	274,234,000	276,352,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	278,986,000
Total at end of period	278,986,000	267,660,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	62,388,000	65,704,000
12 months or greater	11,852,000	163,272,000
Total	74,240,000	228,976,000
Less than 12 months	(728,000)	(1,060,000)
12 months or greater	(51,000)	(8,520,000)
Total	(779,000)	(9,580,000)
CLOs
Debt Securities, Available-for-sale [Line Items]
Total at end of period	327,076,000	310,587,000
Gross Unrealized Gains	33,000	2,000
Gross Unrealized Losses	(1,643,000)	(5,294,000)
Total at end of period	325,466,000	305,295,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	327,076,000
Total at end of period	327,076,000	310,587,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	325,466,000
Total at end of period	325,466,000	305,295,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	81,444,000	296,497,000
12 months or greater	196,988,000	0
Total	278,432,000	296,497,000
Less than 12 months	(225,000)	(5,294,000)
12 months or greater	(1,418,000)	0
Total	(1,643,000)	(5,294,000)
Equity securities
Equity Securities, FV-NI, Gain (Loss) [Abstract]
Equity securities, cost	17,188,000	3,993,000
Gross gains	154,000	11,000
Gross losses	(14,000)	(72,000)
Equity securities, at fair value (cost, 2019 - $17,188; 2018 - $3,993)	17,328,000	3,932,000
Proceeds from sale of equity securities	1,700,000	4,900,000
Gross realized gains on equity securities	1,600,000	3,700,000
Recognized net losses on equity securities still held	$ 201,000	$ 84,000