Accumulated Other Comprehensive Income (Loss) - Changes in AOCI, net of tax (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 30, 2017		Sep. 30, 2016	Sep. 30, 2017		Sep. 30, 2016		Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax		$ 921,165		$ 1,408,587	$ 715,546		$ 686,616
OCI before reclassifications		137,490		252,673	512,030		1,275,869
Amounts reclassified from AOCI		(62,438)		(149,032)	(206,713)		(374,490)
Tax benefit (expense)		(27,289)	[1]	7,577	(51,935)	[1]	(68,190)
Comprehensive income (loss)		47,763		111,218	253,382		833,189
Ending balance, net of tax		968,928		1,519,805	968,928		1,519,805
Change in adjustments related to future policy benefit reserves, net of tax		(5,412)		(2,058)	$ (13,981)		(27,820)
Immaterial Error Correction					17.4
Unrealized gains (losses) on investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax		893,292	[2]	1,475,078	$ 730,429	[2]	803,094	[2]
OCI before reclassifications		112,881	[2]	203,451	442,069	[2]	1,171,094	[2]
Amounts reclassified from AOCI		(64,136)	[2]	(150,482)	(211,281)	[2]	(379,974)	[2]
Tax benefit (expense)		(738)	[1],[2]	8,600	(19,918)	[1],[2]	(57,567)	[2]
Comprehensive income (loss)		48,007	[2]	61,569	210,870	[2]	733,553	[2]
Ending balance, net of tax	[2]	941,299		1,536,647	941,299		1,536,647
Cumulative impact of net unrealized gains losses on future policy benefit reserve		198,100			198,100				$ 212,100
Change in adjustments related to future policy benefit reserves, net of tax					(14,000)
OTTI losses recognized in AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax		(48,013)		(53,383)	(50,952)		(57,502)
OCI before reclassifications		0		0	0		0
Amounts reclassified from AOCI		1,746		1,497	4,712		5,618
Tax benefit (expense)		(25)	[1]	(36)	(52)	[1]	(38)
Comprehensive income (loss)		1,721		1,461	4,660		5,580
Ending balance, net of tax		(46,292)		(51,922)	(46,292)		(51,922)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax		99,816		10,401	58,970		(36,503)
OCI before reclassifications		24,882		45,185	71,114		101,687
Amounts reclassified from AOCI		0		0	0		0
Tax benefit (expense)		(26,491)	[1]	(987)	(31,877)	[1]	(10,585)
Comprehensive income (loss)		(1,609)		44,198	39,237		91,102
Ending balance, net of tax		98,207		54,599	98,207		54,599
Underfunded pension liability
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax		(25,820)		(25,590)	(24,887)		(24,641)
OCI before reclassifications		(273)		4,037	(1,153)		3,088
Amounts reclassified from AOCI		0		0	0		0
Tax benefit (expense)		(35)	[1]	0	(88)	[1]	0
Comprehensive income (loss)		(308)		4,037	(1,241)		3,088
Ending balance, net of tax		(26,128)		(21,553)	(26,128)		(21,553)
Cash flow hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax		1,890		2,081	1,986		2,168
OCI before reclassifications		0		0	0		0
Amounts reclassified from AOCI		(48)		(47)	(144)		(134)
Tax benefit (expense)		0	[1]	0	0	[1]	0
Comprehensive income (loss)		(48)		(47)	(144)		(134)
Ending balance, net of tax		$ 1,842		$ 2,034	$ 1,842		$ 2,034

[1] During the three months ended September 30, 2017, the Company recorded an out-of-period reduction of $17.4 million to "Foreign currency translation adjustments, net of tax," within the Unaudited 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 quarter is not material to the Unaudited Consolidated Financial Statements, nor is it material to any prior period consolidated financial statements.

[2] 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. At December 31, 2016, the cumulative impact of the Shadow Adjustments was $212.1 million. During the nine months ended September 30, 2017, net movements of $(14.0) million were recorded, resulting in a total cumulative net impact of Shadow Adjustments on future policy benefit reserves of $198.1 million at September 30, 2017.