Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 1,370,776	$ 1,928,598
Less than Twelve Months, Gross Unrealized Losses	38,459	45,616
Twelve Months or More, Fair Value	357,918	408,904
Twelve Months or More, Gross Unrealized Losses	10,905	10,071
Total, Fair Value	1,728,694	2,337,502
Total, Gross Unrealized Losses	49,364	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	536,328	1,336,007
Less than Twelve Months, Gross Unrealized Losses	12,705	39,456
Twelve Months or More, Fair Value	0	5,855
Twelve Months or More, Gross Unrealized Losses	0	10
Total, Fair Value	536,328	1,341,862
Total, Gross Unrealized Losses	12,705	39,466
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	13,690	97,752
Less than Twelve Months, Gross Unrealized Losses	256	436
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	13,690	97,752
Total, Gross Unrealized Losses	256	436
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	252	804
Less than Twelve Months, Gross Unrealized Losses	6	13
Twelve Months or More, Fair Value	79	132
Twelve Months or More, Gross Unrealized Losses	13	7
Total, Fair Value	331	936
Total, Gross Unrealized Losses	19	20
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	264,213	93,147
Less than Twelve Months, Gross Unrealized Losses	7,027	870
Twelve Months or More, Fair Value	2,562	15,491
Twelve Months or More, Gross Unrealized Losses	365	461
Total, Fair Value	266,775	108,638
Total, Gross Unrealized Losses	7,392	1,331
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	73,223	82,709
Less than Twelve Months, Gross Unrealized Losses	4,579	2,111
Twelve Months or More, Fair Value	6,684	59,797
Twelve Months or More, Gross Unrealized Losses	1,535	1,232
Total, Fair Value	79,907	142,506
Total, Gross Unrealized Losses	6,114	3,343
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	76,443	50,150
Less than Twelve Months, Gross Unrealized Losses	7,032	113
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	76,443	50,150
Total, Gross Unrealized Losses	7,032	113
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	176,274	97,414
Less than Twelve Months, Gross Unrealized Losses	5,588	1,652
Twelve Months or More, Fair Value	75,774	91,863
Twelve Months or More, Gross Unrealized Losses	6,286	6,343
Total, Fair Value	252,048	189,277
Total, Gross Unrealized Losses	11,874	7,995
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	223,101	103,911
Less than Twelve Months, Gross Unrealized Losses	1,247	717
Twelve Months or More, Fair Value	272,737	235,759
Twelve Months or More, Gross Unrealized Losses	2,703	2,017
Total, Fair Value	495,838	339,670
Total, Gross Unrealized Losses	3,950	2,734
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	7,252	66,071
Less than Twelve Months, Gross Unrealized Losses	19	236
Twelve Months or More, Fair Value	0	0
Twelve Months or More, Gross Unrealized Losses	0	0
Total, Fair Value	7,252	66,071
Total, Gross Unrealized Losses	19	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	82	7
Twelve Months or More, Gross Unrealized Losses	3	1
Total, Fair Value	82	640
Total, Gross Unrealized Losses	$ 3	$ 13