Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 261,397	$ 1,996,758
Less than Twelve Months, Gross Unrealized Losses	5,827	81,764
Twelve Months or More, Fair Value	308,357	947,390
Twelve Months or More, Gross Unrealized Losses	31,585	60,301
Total, Fair Value	569,754	2,944,148
Total, Gross Unrealized Losses	37,412	142,065
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	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	554	77,055
Less than Twelve Months, Gross Unrealized Losses	0	3,184
Twelve Months or More, Fair Value	652	59,700
Twelve Months or More, Gross Unrealized Losses	1	5,015
Total, Fair Value	1,206	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	33,258	784,916
Less than Twelve Months, Gross Unrealized Losses	297	37,635
Twelve Months or More, Fair Value	42,115	213,147
Twelve Months or More, Gross Unrealized Losses	2,847	16,766
Total, Fair Value	75,373	998,063
Total, Gross Unrealized Losses	3,144	54,401
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	28,929	263,934
Less than Twelve Months, Gross Unrealized Losses	2,123	9,159
Twelve Months or More, Fair Value	35,908	287,031
Twelve Months or More, Gross Unrealized Losses	745	9,432
Total, Fair Value	64,837	550,965
Total, Gross Unrealized Losses	2,868	18,591
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	13,759	124,764
Less than Twelve Months, Gross Unrealized Losses	55	6,286
Twelve Months or More, Fair Value	9,846	72,725
Twelve Months or More, Gross Unrealized Losses	884	5,865
Total, Fair Value	23,605	197,489
Total, Gross Unrealized Losses	939	12,151
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	101,087	424,921
Less than Twelve Months, Gross Unrealized Losses	3,108	22,605
Twelve Months or More, Fair Value	197,638	127,201
Twelve Months or More, Gross Unrealized Losses	26,987	17,417
Total, Fair Value	298,725	552,122
Total, Gross Unrealized Losses	30,095	40,022
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	74,051	112,527
Less than Twelve Months, Gross Unrealized Losses	234	650
Twelve Months or More, Fair Value	21,475	6,523
Twelve Months or More, Gross Unrealized Losses	112	100
Total, Fair Value	95,526	119,050
Total, Gross Unrealized Losses	346	750
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	6,763	49,616
Less than Twelve Months, Gross Unrealized Losses	4	434
Twelve Months or More, Fair Value	0	116,786
Twelve Months or More, Gross Unrealized Losses	0	4,093
Total, Fair Value	6,763	166,402
Total, Gross Unrealized Losses	4	4,527
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	2,996	34,249
Less than Twelve Months, Gross Unrealized Losses	6	240
Twelve Months or More, Fair Value	723	32,432
Twelve Months or More, Gross Unrealized Losses	9	827
Total, Fair Value	3,719	66,681
Total, Gross Unrealized Losses	$ 15	$ 1,067