Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 2,270,394	$ 753,811
Less than Twelve Months, Gross Unrealized Losses	34,702	17,810
Twelve Months or More, Fair Value	32,433	315,767
Twelve Months or More, Gross Unrealized Losses	248	6,072
Total, Fair Value	2,302,827	1,069,578
Total, Gross Unrealized Losses	34,950	23,882
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	74,672	244,507
Less than Twelve Months, Gross Unrealized Losses	97	12,686
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	74,672	244,507
Total, Gross Unrealized Losses	97	12,686
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	8,890	9,440
Less than Twelve Months, Gross Unrealized Losses	705	156
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	8,890	9,440
Total, Gross Unrealized Losses	705	156
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	19,170	257
Less than Twelve Months, Gross Unrealized Losses	510	1
Twelve Months or More, Fair Value	83	87
Twelve Months or More, Gross Unrealized Losses	7	3
Total, Fair Value	19,253	344
Total, Gross Unrealized Losses	517	4
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	784,696	117,755
Less than Twelve Months, Gross Unrealized Losses	12,563	1,335
Twelve Months or More, Fair Value	1,974	2,185
Twelve Months or More, Gross Unrealized Losses	68	26
Total, Fair Value	786,670	119,940
Total, Gross Unrealized Losses	12,631	1,361
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	156,480	35,411
Less than Twelve Months, Gross Unrealized Losses	4,065	2,614
Twelve Months or More, Fair Value	2,482	16,071
Twelve Months or More, Gross Unrealized Losses	46	2,092
Total, Fair Value	158,962	51,482
Total, Gross Unrealized Losses	4,111	4,706
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	331,329	69,610
Less than Twelve Months, Gross Unrealized Losses	3,268	408
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	331,329	69,610
Total, Gross Unrealized Losses	3,268	408
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	350,529	11,679
Less than Twelve Months, Gross Unrealized Losses	7,300	188
Twelve Months or More, Fair Value	2,914	50,809
Twelve Months or More, Gross Unrealized Losses	108	2,663
Total, Fair Value	353,443	62,488
Total, Gross Unrealized Losses	7,408	2,851
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	319,038	219,535
Less than Twelve Months, Gross Unrealized Losses	874	320
Twelve Months or More, Fair Value	24,904	246,535
Twelve Months or More, Gross Unrealized Losses	17	1,285
Total, Fair Value	343,942	466,070
Total, Gross Unrealized Losses	891	1,605
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	225,557	45,617
Less than Twelve Months, Gross Unrealized Losses	5,319	102
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	225,557	45,617
Total, Gross Unrealized Losses	5,319	102
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	33	0
Less than Twelve Months, Gross Unrealized Losses	1	0
Twelve Months or More, Fair Value	76	80
Twelve Months or More, Gross Unrealized Losses	2	3
Total, Fair Value	109	80
Total, Gross Unrealized Losses	$ 3	$ 3