Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	$ 389,638	$ 1,996,758
Less than Twelve Months, Gross Unrealized Losses	10,717	81,764
Twelve Months or More, Fair Value	1,064,376	947,390
Twelve Months or More, Gross Unrealized Losses	49,800	60,301
Total, Fair Value	1,454,014	2,944,148
Total, Gross Unrealized Losses	60,517	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	6,771	31,215
Twelve Months or More, Gross Unrealized Losses	18	780
Total, Fair Value	6,771	155,991
Total, Gross Unrealized Losses	18	2,351
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	4,181	77,055
Less than Twelve Months, Gross Unrealized Losses	28	3,184
Twelve Months or More, Fair Value	71,392	59,700
Twelve Months or More, Gross Unrealized Losses	2,460	5,015
Total, Fair Value	75,573	136,755
Total, Gross Unrealized Losses	2,488	8,199
U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	110,017	784,916
Less than Twelve Months, Gross Unrealized Losses	2,347	37,635
Twelve Months or More, Fair Value	330,357	213,147
Twelve Months or More, Gross Unrealized Losses	15,017	16,766
Total, Fair Value	440,374	998,063
Total, Gross Unrealized Losses	17,364	54,401
U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	54,835	263,934
Less than Twelve Months, Gross Unrealized Losses	4,183	9,159
Twelve Months or More, Fair Value	194,614	287,031
Twelve Months or More, Gross Unrealized Losses	5,002	9,432
Total, Fair Value	249,449	550,965
Total, Gross Unrealized Losses	9,185	18,591
Foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	7,508	124,764
Less than Twelve Months, Gross Unrealized Losses	93	6,286
Twelve Months or More, Fair Value	83,890	72,725
Twelve Months or More, Gross Unrealized Losses	3,860	5,865
Total, Fair Value	91,398	197,489
Total, Gross Unrealized Losses	3,953	12,151
Foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	114,485	424,921
Less than Twelve Months, Gross Unrealized Losses	3,592	22,605
Twelve Months or More, Fair Value	228,928	127,201
Twelve Months or More, Gross Unrealized Losses	21,624	17,417
Total, Fair Value	343,413	552,122
Total, Gross Unrealized Losses	25,216	40,022
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	98,426	112,527
Less than Twelve Months, Gross Unrealized Losses	473	650
Twelve Months or More, Fair Value	10,015	6,523
Twelve Months or More, Gross Unrealized Losses	58	100
Total, Fair Value	108,441	119,050
Total, Gross Unrealized Losses	531	750
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	0	49,616
Less than Twelve Months, Gross Unrealized Losses	0	434
Twelve Months or More, Fair Value	110,228	116,786
Twelve Months or More, Gross Unrealized Losses	1,379	4,093
Total, Fair Value	110,228	166,402
Total, Gross Unrealized Losses	1,379	4,527
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months, Fair Value	186	34,249
Less than Twelve Months, Gross Unrealized Losses	1	240
Twelve Months or More, Fair Value	28,181	32,432
Twelve Months or More, Gross Unrealized Losses	382	827
Total, Fair Value	28,367	66,681
Total, Gross Unrealized Losses	$ 383	$ 1,067