Accumulated Other Comprehensive Income - Changes in AOCI, net of tax (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, net of tax, beginning of period		$ 715,546		$ 686,616	$ 1,484,458
OCI before reclassifications		477,907		355,773	(514,094)
Amounts reclassified from AOCI		(291,429)		(344,393)	(326,979)
Tax benefit (expense)		(12,593)		17,550	43,231
Net current period OCI - net of tax		173,885		28,930	(797,842)
Balance, net of tax, end of period		889,431		715,546	686,616
Cumulative adjustments recorded to value of future policy benefit reserves, amount		(27,184)		(62,295)	(170,688)
Unrealized Gains (Losses) on Investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, net of tax, beginning of period	[1]	730,429		803,094	1,590,114
OCI before reclassifications	[1]	449,432		254,333	(474,304)
Amounts reclassified from AOCI	[1]	(298,852)		(353,910)	(346,911)
Tax benefit (expense)	[1]	(10,531)		26,912	34,195
Net current period OCI - net of tax	[1]	140,049		(72,665)	(787,020)
Balance, net of tax, end of period	[1]	870,478		730,429	803,094
Cumulative adjustments recorded to value of future policy benefit reserves, amount		(27,184)
Cumulative impact of net unrealized gains (losses) on future policy benefit reserve		184,900		212,100
OTTI Losses Recognized in AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, net of tax, beginning of period		(50,952)		(57,502)	(76,047)
OCI before reclassifications		0		0	0
Amounts reclassified from AOCI		7,488		6,599	18,987
Tax benefit (expense)		(63)		(49)	(442)
Net current period OCI - net of tax		7,425		6,550	18,545
Balance, net of tax, end of period		(43,527)		(50,952)	(57,502)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, net of tax, beginning of period		58,970		(36,503)	(11,188)
OCI before reclassifications		27,740		102,772	(33,690)
Amounts reclassified from AOCI		0		0	0
Tax benefit (expense)		(3,668)	[2]	(7,299)	8,375
Net current period OCI - net of tax		24,072		95,473	(25,315)
Balance, net of tax, end of period		83,042		58,970	(36,503)
Underfunded Pension Liability
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, net of tax, beginning of period		(24,887)		(24,641)	(20,789)
OCI before reclassifications		735		(1,332)	(6,100)
Amounts reclassified from AOCI		127		3,100	1,145
Tax benefit (expense)		1,669		(2,014)	1,103
Net current period OCI - net of tax		2,531		(246)	(3,852)
Balance, net of tax, end of period		(22,356)		(24,887)	(24,641)
Cash Flow Hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, net of tax, beginning of period		1,986		2,168	2,368
OCI before reclassifications		0		0	0
Amounts reclassified from AOCI		(192)		(182)	(200)
Tax benefit (expense)		0		0	0
Net current period OCI - net of tax		(192)		(182)	(200)
Balance, net of tax, end of period		$ 1,794		$ 1,986	$ 2,168
Comprehensive Income [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Immaterial Error Correction		17.4

[1] For certain annuity contracts that are subject to the GreyCastle Life Retro Arrangements, policy benefit reserves were historically increased for the impact of changes in unrealized gains on investments supporting such contracts as if the gains had been realized, with a corresponding entry to other comprehensive income ("Shadow Adjustments"). Upon completion of the GreyCastle Life Retro Arrangements, no further Shadow Adjustments were recorded. As of December 31, 2016, the cumulative impact of the Shadow Adjustments was $212.1 million. During the year ended December 31, 2017, net movement of $(27.2) million were recorded, resulting in a total cumulative net impact of Shadow Adjustments on future policy benefit reserves of $184.9 million at December 31, 2017.

[2] During the the year ended December 31, 2017, the Company recorded an out-of-period reduction of $17.4 million to "Foreign currency translation adjustments, net of tax," within the Consolidated Statements of Comprehensive Income. This adjustment arose from a previously unrecognized tax liability on foreign exchange movements that should have been reflected in the comprehensive income of a prior period. The Company has evaluated the quantitative and qualitative aspects of this correction and concluded that the impact of recognizing it during this year is not material to the Consolidated Financial Statements, nor is it material to any prior period Consolidated Financial Statements.