Accumulated Other Comprehensive Income (Loss) - Changes in AOCI, net of tax (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax		$ 889,431	$ 715,546
OCI before reclassifications		(403,127)	188,822
Amounts reclassified from AOCI		14,540	(51,147)
Tax benefit (expense)		29,364	(8,247)
Comprehensive income (loss)		(359,223)	129,428
Ending balance, net of tax		312,255	844,974
Change in adjustments related to future policy benefit reserves, net of tax		(4,164)	(5,132)
Cumulative Effect on Retained Earnings, before Tax		233,800
Cumulative Effect on Retained Earnings, Tax		(11,900)
Unrealized gains (losses) on investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax	[1]	870,478	730,429
OCI before reclassifications	[1]	(399,792)	149,959
Amounts reclassified from AOCI	[1]	10,875	(52,554)
Tax benefit (expense)	[1]	28,747	(10,952)
Comprehensive income (loss)	[1]	(360,170)	86,453
Ending balance, net of tax	[1]	294,809	816,882
Cumulative impact of net unrealized gains losses on future policy benefit reserve		180,800		$ 184,900
Change in adjustments related to future policy benefit reserves, net of tax		(4,200)
OTTI losses recognized in AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax		(43,527)	(50,952)
OCI before reclassifications		0	0
Amounts reclassified from AOCI		3,713	1,455
Tax benefit (expense)		(97)	(13)
Comprehensive income (loss)		3,616	1,442
Ending balance, net of tax		(39,911)	(49,510)
Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax		83,042	58,970
OCI before reclassifications		(3,112)	39,204
Amounts reclassified from AOCI		0	0
Tax benefit (expense)		680	2,734
Comprehensive income (loss)		(2,432)	41,938
Ending balance, net of tax		80,394	100,908
Underfunded pension liability
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax		(22,356)	(24,887)
OCI before reclassifications		(223)	(341)
Amounts reclassified from AOCI		0	0
Tax benefit (expense)		34	(16)
Comprehensive income (loss)		(189)	(357)
Ending balance, net of tax		(24,783)	(25,244)
Cash flow hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance, net of tax		1,794	1,986
OCI before reclassifications		0	0
Amounts reclassified from AOCI		(48)	(48)
Tax benefit (expense)		0	0
Comprehensive income (loss)		(48)	(48)
Ending balance, net of tax		1,746	$ 1,938
Accounting Standards Update 2016-01
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement	[2]	221,856
Accounting Standards Update 2016-01 | Unrealized gains (losses) on investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement	[1],[2]	221,856
Accounting Standards Update 2016-01 | OTTI losses recognized in AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement	[2]	0
Accounting Standards Update 2016-01 | Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement	[2]	0
Accounting Standards Update 2016-01 | Underfunded pension liability
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement	[2]	0
Accounting Standards Update 2016-01 | Cash flow hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement	[2]	0
Accounting Standards Update 2018-02
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement		(3,903)
Accounting Standards Update 2018-02 | Unrealized gains (losses) on investments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement	[1]	(6,357)
Accounting Standards Update 2018-02 | OTTI losses recognized in AOCI
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement		0
Accounting Standards Update 2018-02 | Foreign currency translation adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement		216
Accounting Standards Update 2018-02 | Underfunded pension liability
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement		2,238
Accounting Standards Update 2018-02 | Cash flow hedge
Accumulated Other Comprehensive Income (Loss) [Line Items]
Reclassification due to adoption of new accounting pronouncement		$ 0

[1] For certain annuity contracts that are subject to the GreyCastle Life Retro Arrangements, future 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, 2017, the cumulative impact of the Shadow Adjustments was $184.9 million. During the three months ended March 31, 2018, net movements of $(4.2) million were recorded, resulting in a total cumulative net impact of Shadow Adjustments on future policy benefit reserves of $180.8 million at March 31, 2018.

[2] The reclassification out of AOCI due to the adoption of ASU 2016-01 includes $233.8 million of net unrealized gains less $11.9 million of deferred tax on net unrealized gains as discussed in Note 1(b), "Significant Accounting Policies - Recent Accounting Pronouncements."