Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 753,811	$ 1,928,598
Less than Twelve Months, Gross Unrealized Losses	17,810	45,616
Twelve Months or More, Fair Value	315,767	408,904
Twelve Months or More, Gross Unrealized Losses	6,072	10,071
Total, Fair Value	1,069,578	2,337,502
Total, Gross Unrealized Losses	23,882	55,687
U.S. Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	244,507	1,336,007
Less than Twelve Months, Gross Unrealized Losses	12,686	39,456
Twelve Months or More, Fair Value	0	5,855
Twelve Months or More, Gross Unrealized Losses	0	10
Total, Fair Value	244,507	1,341,862
Total, Gross Unrealized Losses	12,686	39,466
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	9,440	97,752
Less than Twelve Months, Gross Unrealized Losses	156	436
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	9,440	97,752
Total, Gross Unrealized Losses	156	436
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	257	804
Less than Twelve Months, Gross Unrealized Losses	1	13
Twelve Months or More, Fair Value	87	132
Twelve Months or More, Gross Unrealized Losses	3	7
Total, Fair Value	344	936
Total, Gross Unrealized Losses	4	20
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	117,755	93,147
Less than Twelve Months, Gross Unrealized Losses	1,335	870
Twelve Months or More, Fair Value	2,185	15,491
Twelve Months or More, Gross Unrealized Losses	26	461
Total, Fair Value	119,940	108,638
Total, Gross Unrealized Losses	1,361	1,331
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	35,411	82,709
Less than Twelve Months, Gross Unrealized Losses	2,614	2,111
Twelve Months or More, Fair Value	16,071	59,797
Twelve Months or More, Gross Unrealized Losses	2,092	1,232
Total, Fair Value	51,482	142,506
Total, Gross Unrealized Losses	4,706	3,343
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	69,610	50,150
Less than Twelve Months, Gross Unrealized Losses	408	113
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	69,610	50,150
Total, Gross Unrealized Losses	408	113
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	11,679	97,414
Less than Twelve Months, Gross Unrealized Losses	188	1,652
Twelve Months or More, Fair Value	50,809	91,863
Twelve Months or More, Gross Unrealized Losses	2,663	6,343
Total, Fair Value	62,488	189,277
Total, Gross Unrealized Losses	2,851	7,995
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	219,535	103,911
Less than Twelve Months, Gross Unrealized Losses	320	717
Twelve Months or More, Fair Value	246,535	235,759
Twelve Months or More, Gross Unrealized Losses	1,285	2,017
Total, Fair Value	466,070	339,670
Total, Gross Unrealized Losses	1,605	2,734
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	45,617	66,071
Less than Twelve Months, Gross Unrealized Losses	102	236
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	45,617	66,071
Total, Gross Unrealized Losses	102	236
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	633
Less than Twelve Months, Gross Unrealized Losses	0	12
Twelve Months or More, Fair Value	80	7
Twelve Months or More, Gross Unrealized Losses	3	1
Total, Fair Value	80	640
Total, Gross Unrealized Losses	$ 3	$ 13