Investments (Details)	12 Months Ended
	Dec. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Total at end of period	$ 6,397,658,000	$ 6,317,164,000
Total at end of period	6,587,581,000	6,661,596,000
Assets held by insurance regulatory requirements	13,400,000	14,100,000
Assets held in trust for the benefit of contractual counterparties	$ 189,800,000	165,900,000
Collateral pledged to principal balance of FHLB advance (as a percent)	102.00%
Collateral pledged for FHLB advance	$ 167,200,000
Maturities, Amortized Cost [Abstract]
Due in one year or less	338,988,000
Due after one year through five years	1,751,297,000
Due after five years through ten years	1,745,569,000
Due after ten years	1,425,159,000
Total debt securities with single maturity date	5,261,013,000
Total at end of period	6,397,658,000	6,317,164,000
Maturities, Fair Value [Abstract]
Due in one year or less	341,604,000
Due after one year through five years	1,795,249,000
Due after five years through ten years	1,824,307,000
Due after ten years	1,486,532,000
Total debt securities with single maturity date	5,447,692,000
Total at end of period	6,587,581,000	6,661,596,000
Equity Securities, FV-NI, Gain (Loss) [Abstract]
Equity securities, cost	15,838,000	17,522,000
Equity securities, at fair value (cost, 2021 - $15,838; 2020 - $17,522)	16,068,000	18,215,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	1,970,111,000	300,911,000
12 months or greater	85,830,000	80,269,000
Total	2,055,941,000	381,180,000
Less than 12 months	(26,842,000)	(3,827,000)
12 months or greater	(1,625,000)	(832,000)
Total	$ (28,467,000)	$ (4,659,000)
Number of securities in unrealized loss position | security	610	109
For securities in an unrealized loss position, percentage of fair value to amortized cost (in hundredths)	99.00%	99.00%
For securities in an unrealized loss position, percentage backed by the U.S. Government (in hundredths)	15.00%	27.00%
Total fixed income securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	$ 6,397,658,000	$ 6,317,164,000
Allowance for Expected Credit Loss	0	(49,000)
Gross Unrealized Gains	218,390,000	349,140,000
Gross Unrealized Losses	(28,467,000)	(4,659,000)
Total at end of period	6,587,581,000	6,661,596,000
Maturities, Amortized Cost [Abstract]
Total at end of period	6,397,658,000	6,317,164,000
Maturities, Fair Value [Abstract]
Total at end of period	6,587,581,000	6,661,596,000
Proceeds from sale of fixed income securities available for sale	471,800,000	803,400,000	$ 228,100,000
Gross realized gains on sale of fixed income securities	9,000,000	21,300,000	7,100,000
Gross realized losses on sale of fixed income securities	1,900,000	8,800,000	3,500,000
Realized losses on fixed income securities related to our intent to sell	0	300,000
Increase in credit allowance for the period		49,000
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	133,990,000	264,531,000
Allowance for Expected Credit Loss	0	0
Gross Unrealized Gains	285,000	1,164,000
Gross Unrealized Losses	(868,000)	(2,000)
Total at end of period	133,407,000	265,693,000
Maturities, Amortized Cost [Abstract]
Total at end of period	133,990,000	264,531,000
Maturities, Fair Value [Abstract]
Total at end of period	133,407,000	265,693,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	91,154,000	2,690,000
12 months or greater	2,616,000	0
Total	93,770,000	2,690,000
Less than 12 months	(790,000)	(2,000)
12 months or greater	(78,000)	0
Total	(868,000)	(2,000)
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Total at end of period	2,408,688,000	2,083,568,000
Allowance for Expected Credit Loss	0	0
Gross Unrealized Gains	133,361,000	166,557,000
Gross Unrealized Losses	(7,396,000)	(256,000)
Total at end of period	2,534,653,000	2,249,869,000
Maturities, Amortized Cost [Abstract]
Total at end of period	2,408,688,000	2,083,568,000
Maturities, Fair Value [Abstract]
Total at end of period	2,534,653,000	2,249,869,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	452,021,000	31,416,000
12 months or greater	15,540,000	0
Total	467,561,000	31,416,000
Less than 12 months	(7,189,000)	(256,000)
12 months or greater	(207,000)	0
Total	(7,396,000)	(256,000)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	2,704,586,000	2,690,860,000
Allowance for Expected Credit Loss	0	0
Gross Unrealized Gains	75,172,000	155,156,000
Gross Unrealized Losses	(13,776,000)	(1,728,000)
Total at end of period	2,765,982,000	2,844,288,000
Maturities, Amortized Cost [Abstract]
Total at end of period	2,704,586,000	2,690,860,000
Maturities, Fair Value [Abstract]
Total at end of period	2,765,982,000	2,844,288,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	865,085,000	44,968,000
12 months or greater	10,997,000	0
Total	876,082,000	44,968,000
Less than 12 months	(13,260,000)	(1,728,000)
12 months or greater	(516,000)	0
Total	(13,776,000)	(1,728,000)
ABS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	150,888,000	203,807,000
Allowance for Expected Credit Loss	0	(49,000)
Gross Unrealized Gains	830,000	2,946,000
Gross Unrealized Losses	(1,008,000)	(18,000)
Total at end of period	150,710,000	206,686,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	150,888,000
Total at end of period	150,888,000	203,807,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	150,710,000
Total at end of period	150,710,000	206,686,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	100,064,000	14,929,000
12 months or greater	1,552,000	0
Total	101,616,000	14,929,000
Less than 12 months	(998,000)	(18,000)
12 months or greater	(10,000)	0
Total	(1,008,000)	(18,000)
RMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	309,991,000	425,532,000
Allowance for Expected Credit Loss	0	0
Gross Unrealized Gains	2,397,000	6,472,000
Gross Unrealized Losses	(3,278,000)	(838,000)
Total at end of period	309,110,000	431,166,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	309,991,000
Total at end of period	309,991,000	425,532,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	309,110,000
Total at end of period	309,110,000	431,166,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	180,586,000	98,409,000
12 months or greater	31,641,000	3,566,000
Total	212,227,000	101,975,000
Less than 12 months	(2,548,000)	(773,000)
12 months or greater	(730,000)	(65,000)
Total	(3,278,000)	(838,000)
CMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	315,330,000	312,572,000
Allowance for Expected Credit Loss	0	0
Gross Unrealized Gains	5,736,000	16,055,000
Gross Unrealized Losses	(1,936,000)	(1,125,000)
Total at end of period	319,130,000	327,502,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	315,330,000
Total at end of period	315,330,000	312,572,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	319,130,000
Total at end of period	319,130,000	327,502,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	89,889,000	13,212,000
12 months or greater	1,511,000	2,799,000
Total	91,400,000	16,011,000
Less than 12 months	(1,887,000)	(789,000)
12 months or greater	(49,000)	(336,000)
Total	(1,936,000)	(1,125,000)
CLOs
Debt Securities, Available-for-sale [Line Items]
Total at end of period	360,436,000	310,616,000
Allowance for Expected Credit Loss	0	0
Gross Unrealized Gains	609,000	566,000
Gross Unrealized Losses	(106,000)	(692,000)
Total at end of period	360,939,000	310,490,000
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	360,436,000
Total at end of period	360,436,000	310,616,000
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	360,939,000
Total at end of period	360,939,000	310,490,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	177,663,000	95,287,000
12 months or greater	21,973,000	73,904,000
Total	199,636,000	169,191,000
Less than 12 months	(71,000)	(261,000)
12 months or greater	(35,000)	(431,000)
Total	(106,000)	(692,000)
Foreign government debt
Debt Securities, Available-for-sale [Line Items]
Total at end of period	13,749,000	4,485,000
Allowance for Expected Credit Loss	0	0
Gross Unrealized Gains	0	224,000
Gross Unrealized Losses	(99,000)	0
Total at end of period	13,650,000	4,709,000
Maturities, Amortized Cost [Abstract]
Total at end of period	13,749,000	4,485,000
Maturities, Fair Value [Abstract]
Total at end of period	13,650,000	4,709,000
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	13,649,000
12 months or greater	0
Total	13,649,000
Less than 12 months	(99,000)
12 months or greater	0
Total	(99,000)
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Total at end of period		21,193,000
Allowance for Expected Credit Loss		0
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Total at end of period		21,193,000
Maturities, Amortized Cost [Abstract]
Total at end of period		21,193,000
Maturities, Fair Value [Abstract]
Total at end of period		21,193,000
Equity securities
Equity Securities, FV-NI, Gain (Loss) [Abstract]
Equity securities, cost	15,838,000	17,522,000
Gross gains	264,000	695,000
Gross losses	(34,000)	(2,000)
Equity securities, at fair value (cost, 2021 - $15,838; 2020 - $17,522)	16,068,000	18,215,000
Proceeds from sale of equity securities	2,600,000	25,700,000	1,700,000
Gross realized gains on equity securities	6,000	1,800,000	1,600,000
Gross realized losses on equity securities	2,300	400,000	0
Recognized net gains (losses) on equity securities still held	$ (500,000)	$ 600,000	$ 200,000