Investments (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($) security	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Total at end of period	$ 6,317,164	$ 5,562,550
Total at end of period	6,661,596	5,737,892
Assets held by insurance regulatory requirements	14,100	13,900
Assets held in trust for the benefit of contractual counterparties	$ 165,900	88,900
Collateral pledged to principal balance of FHLB advance (as a percent)	102.00%
Collateral pledged for FHLB advance	$ 163,900
Maturities, Amortized Cost [Abstract]
Due in one year or less	390,510
Due after one year through five years	1,945,158
Due after five years through ten years	1,280,760
Due after ten years	1,448,209
Total debt securities with single maturity date	5,064,637
Total at end of period	6,317,164	5,562,550
Maturities, Fair Value [Abstract]
Due in one year or less	393,286
Due after one year through five years	2,033,183
Due after five years through ten years	1,402,640
Due after ten years	1,556,643
Total debt securities with single maturity date	5,385,752
Total at end of period	6,661,596	5,737,892
Equity Securities, FV-NI, Gain (Loss) [Abstract]
Equity securities, cost	17,522	17,188
Equity securities, at fair value (cost, 2020 - $17,522; 2019 - $17,188)	18,215	17,328
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	300,911	624,334
12 months or greater	80,269	379,411
Total	381,180	1,003,745
Less than 12 months	(3,827)	(5,426)
12 months or greater	(832)	(4,476)
Total	$ (4,659)	$ (9,902)
Number of securities in unrealized loss position | security	109	217
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 fixed income securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	$ 6,317,164	$ 5,562,550
Allowance for Expected Credit Loss	(49)
Gross Unrealized Gains	349,140	185,244
Gross Unrealized Losses	(4,659)	(9,902)
Total at end of period	6,661,596	5,737,892
Maturities, Amortized Cost [Abstract]
Total at end of period	6,317,164	5,562,550
Maturities, Fair Value [Abstract]
Total at end of period	6,661,596	5,737,892
Proceeds from sale of fixed income securities available for sale	803,400	228,100	$ 365,600
Gross realized gains	21,300	7,100	700
Gross realized losses	8,800	3,500	3,800
Debt Securities, Available for Sale, Impairment Charges Intent To Sell No Allowance Previously Recorded Recognized in Net Realized Investment Gains Losses	300
Increase in credit allowance for the period	49
Recognized other than temporary impairments		100	$ 1,800
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	264,531	195,176
Allowance for Expected Credit Loss	0
Gross Unrealized Gains	1,164	1,237
Gross Unrealized Losses	(2)	(210)
Total at end of period	265,693	196,203
Maturities, Amortized Cost [Abstract]
Total at end of period	264,531	195,176
Maturities, Fair Value [Abstract]
Total at end of period	265,693	196,203
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	2,690	57,301
12 months or greater	0	5,806
Total	2,690	63,107
Less than 12 months	(2)	(200)
12 months or greater	0	(10)
Total	(2)	(210)
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Total at end of period	2,083,568	1,555,394
Allowance for Expected Credit Loss	0
Gross Unrealized Gains	166,557	99,328
Gross Unrealized Losses	(256)	(857)
Total at end of period	2,249,869	1,653,865
Maturities, Amortized Cost [Abstract]
Total at end of period	2,083,568	1,555,394
Maturities, Fair Value [Abstract]
Total at end of period	2,249,869	1,653,865
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	31,416	74,859
12 months or greater	0	6,957
Total	31,416	81,816
Less than 12 months	(256)	(847)
12 months or greater	0	(10)
Total	(256)	(857)
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Total at end of period	2,690,860	2,711,910
Allowance for Expected Credit Loss	0
Gross Unrealized Gains	155,156	76,220
Gross Unrealized Losses	(1,728)	(3,008)
Total at end of period	2,844,288	2,785,122
Maturities, Amortized Cost [Abstract]
Total at end of period	2,690,860	2,711,910
Maturities, Fair Value [Abstract]
Total at end of period	2,844,288	2,785,122
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	44,968	221,357
12 months or greater	0	43,505
Total	44,968	264,862
Less than 12 months	(1,728)	(2,847)
12 months or greater	0	(161)
Total	(1,728)	(3,008)
ABS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	203,807	227,376
Allowance for Expected Credit Loss	(49)
Gross Unrealized Gains	2,946	2,466
Gross Unrealized Losses	(18)	(178)
Total at end of period	206,686	229,664
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	203,807
Total at end of period	203,807	227,376
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	206,686
Total at end of period	206,686	229,664
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	14,929	21,542
12 months or greater	0	3,851
Total	14,929	25,393
Less than 12 months	(18)	(118)
12 months or greater	0	(60)
Total	(18)	(178)
RMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	425,532	271,384
Allowance for Expected Credit Loss	0
Gross Unrealized Gains	6,472	429
Gross Unrealized Losses	(838)	(3,227)
Total at end of period	431,166	268,586
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	425,532
Total at end of period	425,532	271,384
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	431,166
Total at end of period	431,166	268,586
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	98,409	105,443
12 months or greater	3,566	110,452
Total	101,975	215,895
Less than 12 months	(773)	(461)
12 months or greater	(65)	(2,766)
Total	(838)	(3,227)
CMBS
Debt Securities, Available-for-sale [Line Items]
Total at end of period	312,572	274,234
Allowance for Expected Credit Loss	0
Gross Unrealized Gains	16,055	5,531
Gross Unrealized Losses	(1,125)	(779)
Total at end of period	327,502	278,986
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	312,572
Total at end of period	312,572	274,234
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	327,502
Total at end of period	327,502	278,986
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	13,212	62,388
12 months or greater	2,799	11,852
Total	16,011	74,240
Less than 12 months	(789)	(728)
12 months or greater	(336)	(51)
Total	(1,125)	(779)
CLOs
Debt Securities, Available-for-sale [Line Items]
Total at end of period	310,616	327,076
Allowance for Expected Credit Loss	0
Gross Unrealized Gains	566	33
Gross Unrealized Losses	(692)	(1,643)
Total at end of period	310,490	325,466
Maturities, Amortized Cost [Abstract]
Debt securities without single maturity date	310,616
Total at end of period	310,616	327,076
Maturities, Fair Value [Abstract]
Debt securities without single maturity date	310,490
Total at end of period	310,490	325,466
Continuous Unrealized Loss Portion, Unrealized Loses [Abstract]
Less than 12 months	95,287	81,444
12 months or greater	73,904	196,988
Total	169,191	278,432
Less than 12 months	(261)	(225)
12 months or greater	(431)	(1,418)
Total	(692)	(1,643)
Debt securities issued by foreign sovereign governments
Debt Securities, Available-for-sale [Line Items]
Total at end of period	4,485
Allowance for Expected Credit Loss	0
Gross Unrealized Gains	224
Gross Unrealized Losses	0
Total at end of period	4,709
Maturities, Amortized Cost [Abstract]
Total at end of period	4,485
Maturities, Fair Value [Abstract]
Total at end of period	4,709
Other investments
Debt Securities, Available-for-sale [Line Items]
Total at end of period	21,193
Allowance for Expected Credit Loss	0
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Total at end of period	21,193
Maturities, Amortized Cost [Abstract]
Total at end of period	21,193
Maturities, Fair Value [Abstract]
Total at end of period	21,193
Equity securities
Equity Securities, FV-NI, Gain (Loss) [Abstract]
Equity securities, cost	17,522	17,188
Gross gains	695	154
Gross losses	(2)	(14)
Equity securities, at fair value (cost, 2020 - $17,522; 2019 - $17,188)	18,215	17,328
Proceeds from sale of equity securities	25,700	1,700
Gross realized gains on equity securities	1,800	1,600
Gross realized losses on equity securities	400	0
Recognized net gains (losses) on equity securities still held	$ 600	$ 200