Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 1,540,588	$ 211,240
Less than Twelve Months, Gross Unrealized Losses	123,879	3,131
Twelve Months or More, Fair Value	231,061	293,974
Twelve Months or More, Gross Unrealized Losses	51,737	20,712
Total, Fair Value	1,771,649	505,214
Total, Gross Unrealized Losses	175,616	23,843
US Treasury securities and obligations of U.S. government authorities and agencies
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	0
Less than Twelve Months, Gross Unrealized Losses	0	0
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	0	0
Total, Gross Unrealized Losses	0	0
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	2,995	0
Less than Twelve Months, Gross Unrealized Losses	133	0
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	2,995	0
Total, Gross Unrealized Losses	133	0
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	28,153	2,152
Less than Twelve Months, Gross Unrealized Losses	1,431	1
Twelve Months or More, Fair Value	0	400
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	28,153	2,552
Total, Gross Unrealized Losses	1,431	1
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	505,567	81,622
Less than Twelve Months, Gross Unrealized Losses	56,058	984
Twelve Months or More, Fair Value	4,452	19,206
Twelve Months or More, Gross Unrealized Losses	1,572	1,263
Total, Fair Value	510,019	100,828
Total, Gross Unrealized Losses	57,630	2,247
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	257,998	33,264
Less than Twelve Months, Gross Unrealized Losses	16,596	780
Twelve Months or More, Fair Value	6,503	22,143
Twelve Months or More, Gross Unrealized Losses	580	226
Total, Fair Value	264,501	55,407
Total, Gross Unrealized Losses	17,176	1,006
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	75,440	3,839
Less than Twelve Months, Gross Unrealized Losses	6,109	23
Twelve Months or More, Fair Value	4,445	9,379
Twelve Months or More, Gross Unrealized Losses	2,813	362
Total, Fair Value	79,885	13,218
Total, Gross Unrealized Losses	8,922	385
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	533,303	32,800
Less than Twelve Months, Gross Unrealized Losses	38,880	921
Twelve Months or More, Fair Value	174,865	186,693
Twelve Months or More, Gross Unrealized Losses	43,918	18,630
Total, Fair Value	708,168	219,493
Total, Gross Unrealized Losses	82,798	19,551
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	64,751	32,361
Less than Twelve Months, Gross Unrealized Losses	3,689	243
Twelve Months or More, Fair Value	40,796	55,461
Twelve Months or More, Gross Unrealized Losses	2,854	223
Total, Fair Value	105,547	87,822
Total, Gross Unrealized Losses	6,543	466
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	66,503	22,153
Less than Twelve Months, Gross Unrealized Losses	444	163
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	66,503	22,153
Total, Gross Unrealized Losses	444	163
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	5,878	3,049
Less than Twelve Months, Gross Unrealized Losses	539	16
Twelve Months or More, Fair Value	0	692
Twelve Months or More, Gross Unrealized Losses	0	8
Total, Fair Value	5,878	3,741
Total, Gross Unrealized Losses	$ 539	$ 24