Investments (Fair Value and Losses by Investment Category and Length of Time in a Loss Position) (Details) - Fixed maturities - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	$ 2,212,147	$ 297,974
Less than twelve Months, Gross Unrealized Losses	74,511	2,672
Twelve months or more, Fair Value	273,398	419,770
Twelve months or more, Gross Unrealized Losses	29,208	16,170
Total, Fair Value	2,485,545	717,744
Total, Gross Unrealized Losses	103,719	18,842
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	621	0
Less than twelve Months, Gross Unrealized Losses	14	0
Twelve months or more, Fair Value	0	0
Twelve months or more, Gross Unrealized Losses	0	0
Total, Fair Value	621	0
Total, Gross Unrealized Losses	14	0
Obligations of U.S. states and their political subdivisions
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	102,109	0
Less than twelve Months, Gross Unrealized Losses	1,230	0
Twelve months or more, Fair Value	0	0
Twelve months or more, Gross Unrealized Losses	0	0
Total, Fair Value	102,109	0
Total, Gross Unrealized Losses	1,230	0
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	143,401	16,522
Less than twelve Months, Gross Unrealized Losses	7,096	210
Twelve months or more, Fair Value	11,721	11,959
Twelve months or more, Gross Unrealized Losses	1,290	286
Total, Fair Value	155,122	28,481
Total, Gross Unrealized Losses	8,386	496
Public utilities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	283,552	26,466
Less than twelve Months, Gross Unrealized Losses	8,082	380
Twelve months or more, Fair Value	17,233	27,354
Twelve months or more, Gross Unrealized Losses	1,931	678
Total, Fair Value	300,785	53,820
Total, Gross Unrealized Losses	10,013	1,058
Redeemable preferred stock
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	1,791	0
Less than twelve Months, Gross Unrealized Losses	18	0
Twelve months or more, Fair Value	0	0
Twelve months or more, Gross Unrealized Losses	0	0
Total, Fair Value	1,791	0
Total, Gross Unrealized Losses	18	0
All other U.S. public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	600,364	36,100
Less than twelve Months, Gross Unrealized Losses	25,564	304
Twelve months or more, Fair Value	89,649	102,490
Twelve months or more, Gross Unrealized Losses	8,111	2,276
Total, Fair Value	690,013	138,590
Total, Gross Unrealized Losses	33,675	2,580
All other U.S. private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	389,065	79,495
Less than twelve Months, Gross Unrealized Losses	9,928	781
Twelve months or more, Fair Value	41,258	36,642
Twelve months or more, Gross Unrealized Losses	2,520	863
Total, Fair Value	430,323	116,137
Total, Gross Unrealized Losses	12,448	1,644
All other foreign public corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	151,899	27,078
Less than twelve Months, Gross Unrealized Losses	7,925	190
Twelve months or more, Fair Value	6,656	8,390
Twelve months or more, Gross Unrealized Losses	853	542
Total, Fair Value	158,555	35,468
Total, Gross Unrealized Losses	8,778	732
All other foreign private corporate securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	300,641	48,109
Less than twelve Months, Gross Unrealized Losses	10,470	472
Twelve months or more, Fair Value	58,167	133,055
Twelve months or more, Gross Unrealized Losses	11,338	9,126
Total, Fair Value	358,808	181,164
Total, Gross Unrealized Losses	21,808	9,598
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	47,033	6,351
Less than twelve Months, Gross Unrealized Losses	190	22
Twelve months or more, Fair Value	332	317
Twelve months or more, Gross Unrealized Losses	5	4
Total, Fair Value	47,365	6,668
Total, Gross Unrealized Losses	195	26
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	126,345	49,823
Less than twelve Months, Gross Unrealized Losses	2,968	285
Twelve months or more, Fair Value	42,853	93,403
Twelve months or more, Gross Unrealized Losses	2,868	2,254
Total, Fair Value	169,198	143,226
Total, Gross Unrealized Losses	5,836	2,539
Residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than twelve months, Fair Value	65,326	8,030
Less than twelve Months, Gross Unrealized Losses	1,026	28
Twelve months or more, Fair Value	5,529	6,160
Twelve months or more, Gross Unrealized Losses	292	141
Total, Fair Value	70,855	14,190
Total, Gross Unrealized Losses	$ 1,318	$ 169