Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed Maturities - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	$ 1,996,758	$ 297,974
Less than Twelve Months, Gross Unrealized Losses	81,764	2,672
Twelve Months or More Fair Value	947,390	419,770
Twelve Months or More, Gross Unrealized Losses	60,301	16,170
Total Fair Value	2,944,148	717,744
Total Gross Unrealized Losses	142,065	18,842
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	630	0
Twelve Months or More, Gross Unrealized Losses	6	0
Total Fair Value	630	0
Total Gross Unrealized Losses	6	0
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	124,776	0
Less than Twelve Months, Gross Unrealized Losses	1,571	0
Twelve Months or More Fair Value	31,215	0
Twelve Months or More, Gross Unrealized Losses	780	0
Total Fair Value	155,991	0
Total Gross Unrealized Losses	2,351	0
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	77,055	16,522
Less than Twelve Months, Gross Unrealized Losses	3,184	210
Twelve Months or More Fair Value	59,700	11,959
Twelve Months or More, Gross Unrealized Losses	5,015	286
Total Fair Value	136,755	28,481
Total Gross Unrealized Losses	8,199	496
U.S. corporate public securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	784,916	39,213
Less than Twelve Months, Gross Unrealized Losses	37,635	305
Twelve Months or More Fair Value	213,147	107,332
Twelve Months or More, Gross Unrealized Losses	16,766	2,337
Total Fair Value	998,063	146,545
Total Gross Unrealized Losses	54,401	2,642
U.S. corporate private securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	263,934	89,441
Less than Twelve Months, Gross Unrealized Losses	9,159	1,102
Twelve Months or More Fair Value	287,031	40,276
Twelve Months or More, Gross Unrealized Losses	9,432	953
Total Fair Value	550,965	129,717
Total Gross Unrealized Losses	18,591	2,055
Foreign corporate public securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	124,764	27,079
Less than Twelve Months, Gross Unrealized Losses	6,286	190
Twelve Months or More Fair Value	72,725	8,390
Twelve Months or More, Gross Unrealized Losses	5,865	542
Total Fair Value	197,489	35,469
Total Gross Unrealized Losses	12,151	732
Foreign corporate private securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	424,921	61,515
Less than Twelve Months, Gross Unrealized Losses	22,605	530
Twelve Months or More Fair Value	127,201	151,933
Twelve Months or More, Gross Unrealized Losses	17,417	9,653
Total Fair Value	552,122	213,448
Total Gross Unrealized Losses	40,022	10,183
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	112,527	6,351
Less than Twelve Months, Gross Unrealized Losses	650	22
Twelve Months or More Fair Value	6,523	317
Twelve Months or More, Gross Unrealized Losses	100	4
Total Fair Value	119,050	6,668
Total Gross Unrealized Losses	750	26
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	49,616	49,823
Less than Twelve Months, Gross Unrealized Losses	434	285
Twelve Months or More Fair Value	116,786	93,403
Twelve Months or More, Gross Unrealized Losses	4,093	2,254
Total Fair Value	166,402	143,226
Total Gross Unrealized Losses	4,527	2,539
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than Twelve Months Fair Value	34,249	8,030
Less than Twelve Months, Gross Unrealized Losses	240	28
Twelve Months or More Fair Value	32,432	6,160
Twelve Months or More, Gross Unrealized Losses	827	141
Total Fair Value	66,681	14,190
Total Gross Unrealized Losses	$ 1,067	$ 169