Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 3,554,956	$ 753,811
Less than Twelve Months, Gross Unrealized Losses	128,926	17,810
Twelve Months or More, Fair Value	140,918	315,767
Twelve Months or More, Gross Unrealized Losses	7,361	6,072
Total, Fair Value	3,695,874	1,069,578
Total, Gross Unrealized Losses	136,287	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	144,565	244,507
Less than Twelve Months, Gross Unrealized Losses	10,334	12,686
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	144,565	244,507
Total, Gross Unrealized Losses	10,334	12,686
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	79,297	9,440
Less than Twelve Months, Gross Unrealized Losses	2,340	156
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	79,297	9,440
Total, Gross Unrealized Losses	2,340	156
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	18,554	257
Less than Twelve Months, Gross Unrealized Losses	884	1
Twelve Months or More, Fair Value	78	87
Twelve Months or More, Gross Unrealized Losses	12	3
Total, Fair Value	18,632	344
Total, Gross Unrealized Losses	896	4
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	1,864,779	117,755
Less than Twelve Months, Gross Unrealized Losses	70,888	1,335
Twelve Months or More, Fair Value	15,462	2,185
Twelve Months or More, Gross Unrealized Losses	663	26
Total, Fair Value	1,880,241	119,940
Total, Gross Unrealized Losses	71,551	1,361
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	319,025	35,411
Less than Twelve Months, Gross Unrealized Losses	12,473	2,614
Twelve Months or More, Fair Value	16,442	16,071
Twelve Months or More, Gross Unrealized Losses	2,266	2,092
Total, Fair Value	335,467	51,482
Total, Gross Unrealized Losses	14,739	4,706
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	608,178	69,610
Less than Twelve Months, Gross Unrealized Losses	13,056	408
Twelve Months or More, Fair Value	31,663	0
Twelve Months or More, Gross Unrealized Losses	993	0
Total, Fair Value	639,841	69,610
Total, Gross Unrealized Losses	14,049	408
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	212,996	11,679
Less than Twelve Months, Gross Unrealized Losses	8,010	188
Twelve Months or More, Fair Value	43,527	50,809
Twelve Months or More, Gross Unrealized Losses	3,401	2,663
Total, Fair Value	256,523	62,488
Total, Gross Unrealized Losses	11,411	2,851
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	99,710	219,535
Less than Twelve Months, Gross Unrealized Losses	189	320
Twelve Months or More, Fair Value	33,668	246,535
Twelve Months or More, Gross Unrealized Losses	23	1,285
Total, Fair Value	133,378	466,070
Total, Gross Unrealized Losses	212	1,605
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	202,581	45,617
Less than Twelve Months, Gross Unrealized Losses	10,722	102
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	202,581	45,617
Total, Gross Unrealized Losses	10,722	102
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	5,271	0
Less than Twelve Months, Gross Unrealized Losses	30	0
Twelve Months or More, Fair Value	78	80
Twelve Months or More, Gross Unrealized Losses	3	3
Total, Fair Value	5,349	80
Total, Gross Unrealized Losses	$ 33	$ 3