Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 1,538,273	$ 1,928,598
Less than Twelve Months, Gross Unrealized Losses	50,518	45,616
Twelve Months or More, Fair Value	357,070	408,904
Twelve Months or More, Gross Unrealized Losses	19,932	10,071
Total, Fair Value	1,895,343	2,337,502
Total, Gross Unrealized Losses	70,450	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	539,420	1,336,007
Less than Twelve Months, Gross Unrealized Losses	10,467	39,456
Twelve Months or More, Fair Value	0	5,855
Twelve Months or More, Gross Unrealized Losses	0	10
Total, Fair Value	539,420	1,341,862
Total, Gross Unrealized Losses	10,467	39,466
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	3,084	97,752
Less than Twelve Months, Gross Unrealized Losses	5	436
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	3,084	97,752
Total, Gross Unrealized Losses	5	436
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	368	804
Less than Twelve Months, Gross Unrealized Losses	14	13
Twelve Months or More, Fair Value	81	132
Twelve Months or More, Gross Unrealized Losses	11	7
Total, Fair Value	449	936
Total, Gross Unrealized Losses	25	20
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	220,713	93,147
Less than Twelve Months, Gross Unrealized Losses	6,981	870
Twelve Months or More, Fair Value	2,332	15,491
Twelve Months or More, Gross Unrealized Losses	518	461
Total, Fair Value	223,045	108,638
Total, Gross Unrealized Losses	7,499	1,331
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	124,703	82,709
Less than Twelve Months, Gross Unrealized Losses	6,429	2,111
Twelve Months or More, Fair Value	19,376	59,797
Twelve Months or More, Gross Unrealized Losses	1,673	1,232
Total, Fair Value	144,079	142,506
Total, Gross Unrealized Losses	8,102	3,343
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	63,792	50,150
Less than Twelve Months, Gross Unrealized Losses	7,671	113
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	63,792	50,150
Total, Gross Unrealized Losses	7,671	113
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	412,032	97,414
Less than Twelve Months, Gross Unrealized Losses	15,832	1,652
Twelve Months or More, Fair Value	118,199	91,863
Twelve Months or More, Gross Unrealized Losses	11,980	6,343
Total, Fair Value	530,231	189,277
Total, Gross Unrealized Losses	27,812	7,995
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	170,685	103,911
Less than Twelve Months, Gross Unrealized Losses	3,099	717
Twelve Months or More, Fair Value	216,998	235,759
Twelve Months or More, Gross Unrealized Losses	5,747	2,017
Total, Fair Value	387,683	339,670
Total, Gross Unrealized Losses	8,846	2,734
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	3,476	66,071
Less than Twelve Months, Gross Unrealized Losses	20	236
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	3,476	66,071
Total, Gross Unrealized Losses	20	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	84	7
Twelve Months or More, Gross Unrealized Losses	3	1
Total, Fair Value	84	640
Total, Gross Unrealized Losses	$ 3	$ 13