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	$ 2,893,203	$ 1,928,598
Less than Twelve Months, Gross Unrealized Losses	203,194	45,616
Twelve Months or More, Fair Value	281,094	408,904
Twelve Months or More, Gross Unrealized Losses	33,302	10,071
Total, Fair Value	3,174,297	2,337,502
Total, Gross Unrealized Losses	236,496	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	550,574	1,336,007
Less than Twelve Months, Gross Unrealized Losses	5,644	39,456
Twelve Months or More, Fair Value	0	5,855
Twelve Months or More, Gross Unrealized Losses	0	10
Total, Fair Value	550,574	1,341,862
Total, Gross Unrealized Losses	5,644	39,466
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	44,790	97,752
Less than Twelve Months, Gross Unrealized Losses	702	436
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	44,790	97,752
Total, Gross Unrealized Losses	702	436
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	27,964	804
Less than Twelve Months, Gross Unrealized Losses	1,221	13
Twelve Months or More, Fair Value	74	132
Twelve Months or More, Gross Unrealized Losses	19	7
Total, Fair Value	28,038	936
Total, Gross Unrealized Losses	1,240	20
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	437,796	93,147
Less than Twelve Months, Gross Unrealized Losses	37,781	870
Twelve Months or More, Fair Value	2,167	15,491
Twelve Months or More, Gross Unrealized Losses	1,089	461
Total, Fair Value	439,963	108,638
Total, Gross Unrealized Losses	38,870	1,331
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	542,221	82,709
Less than Twelve Months, Gross Unrealized Losses	34,290	2,111
Twelve Months or More, Fair Value	26,844	59,797
Twelve Months or More, Gross Unrealized Losses	2,898	1,232
Total, Fair Value	569,065	142,506
Total, Gross Unrealized Losses	37,188	3,343
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	84,112	50,150
Less than Twelve Months, Gross Unrealized Losses	20,373	113
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	84,112	50,150
Total, Gross Unrealized Losses	20,373	113
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	937,001	97,414
Less than Twelve Months, Gross Unrealized Losses	89,599	1,652
Twelve Months or More, Fair Value	85,795	91,863
Twelve Months or More, Gross Unrealized Losses	16,109	6,343
Total, Fair Value	1,022,796	189,277
Total, Gross Unrealized Losses	105,708	7,995
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	210,569	103,911
Less than Twelve Months, Gross Unrealized Losses	12,622	717
Twelve Months or More, Fair Value	166,207	235,759
Twelve Months or More, Gross Unrealized Losses	13,187	2,017
Total, Fair Value	376,776	339,670
Total, Gross Unrealized Losses	25,809	2,734
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	52,283	66,071
Less than Twelve Months, Gross Unrealized Losses	449	236
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	52,283	66,071
Total, Gross Unrealized Losses	449	236
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	5,893	633
Less than Twelve Months, Gross Unrealized Losses	513	12
Twelve Months or More, Fair Value	7	7
Twelve Months or More, Gross Unrealized Losses	0	1
Total, Fair Value	5,900	640
Total, Gross Unrealized Losses	$ 513	$ 13