Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed Maturities - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	$ 211,240	$ 1,996,758
Less than Twelve Months, Gross Unrealized Losses	3,131	81,764
Twelve Months or More Fair Value	293,974	947,390
Twelve Months or More, Gross Unrealized Losses	20,712	60,301
Total, Fair Value	505,214	2,944,148
Total, Gross Unrealized Losses	23,843	142,065
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	0	0
Less than Twelve Months, Gross Unrealized Losses	0	0
Twelve Months or More Fair Value	0	630
Twelve Months or More, Gross Unrealized Losses	0	6
Total, Fair Value	0	630
Total, Gross Unrealized Losses	0	6
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	0	124,776
Less than Twelve Months, Gross Unrealized Losses	0	1,571
Twelve Months or More Fair Value	0	31,215
Twelve Months or More, Gross Unrealized Losses	0	780
Total, Fair Value	0	155,991
Total, Gross Unrealized Losses	0	2,351
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	2,152	77,055
Less than Twelve Months, Gross Unrealized Losses	1	3,184
Twelve Months or More Fair Value	400	59,700
Twelve Months or More, Gross Unrealized Losses	0	5,015
Total, Fair Value	2,552	136,755
Total, Gross Unrealized Losses	1	8,199
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	81,622	784,916
Less than Twelve Months, Gross Unrealized Losses	984	37,635
Twelve Months or More Fair Value	19,206	213,147
Twelve Months or More, Gross Unrealized Losses	1,263	16,766
Total, Fair Value	100,828	998,063
Total, Gross Unrealized Losses	2,247	54,401
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	33,264	263,934
Less than Twelve Months, Gross Unrealized Losses	780	9,159
Twelve Months or More Fair Value	22,143	287,031
Twelve Months or More, Gross Unrealized Losses	226	9,432
Total, Fair Value	55,407	550,965
Total, Gross Unrealized Losses	1,006	18,591
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	3,839	124,764
Less than Twelve Months, Gross Unrealized Losses	23	6,286
Twelve Months or More Fair Value	9,379	72,725
Twelve Months or More, Gross Unrealized Losses	362	5,865
Total, Fair Value	13,218	197,489
Total, Gross Unrealized Losses	385	12,151
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	32,800	424,921
Less than Twelve Months, Gross Unrealized Losses	921	22,605
Twelve Months or More Fair Value	186,693	127,201
Twelve Months or More, Gross Unrealized Losses	18,630	17,417
Total, Fair Value	219,493	552,122
Total, Gross Unrealized Losses	19,551	40,022
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	32,361	112,527
Less than Twelve Months, Gross Unrealized Losses	243	650
Twelve Months or More Fair Value	55,461	6,523
Twelve Months or More, Gross Unrealized Losses	223	100
Total, Fair Value	87,822	119,050
Total, Gross Unrealized Losses	466	750
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	22,153	49,616
Less than Twelve Months, Gross Unrealized Losses	163	434
Twelve Months or More Fair Value	0	116,786
Twelve Months or More, Gross Unrealized Losses	0	4,093
Total, Fair Value	22,153	166,402
Total, Gross Unrealized Losses	163	4,527
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	3,049	34,249
Less than Twelve Months, Gross Unrealized Losses	16	240
Twelve Months or More Fair Value	692	32,432
Twelve Months or More, Gross Unrealized Losses	8	827
Total, Fair Value	3,741	66,681
Total, Gross Unrealized Losses	$ 24	$ 1,067