Other Comprehensive Income - OCI - Changes in Accumulated Other comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Comprehensive Income [Roll Forward]
Beginning balance			$ 2,449,589	$ 2,012,321
Deferred income tax benefit (expense)	$ (27,251)	$ (26,168)	(40,804)	(43,286)
Ending balance	2,039,996	2,207,290	2,039,996	2,207,290
Interest expense	(9,588)	(7,646)	(17,397)	(15,276)
Unrealized holding gains / losses arising on fixed maturities, available-for- sale
Other Comprehensive Income [Roll Forward]
Beginning balance	286,289	398,293	544,887	311,748
Change in estimate of tax reform impact			108,846
OCI before reclassifications	(353,834)	236,996	(813,514)	367,225
Deferred income tax benefit (expense)	74,304	(82,949)	170,838	(128,529)
AOCI before reclassification, net of tax	(279,530)	154,047	(642,676)	238,696
Amounts reclassified from AOCI	(2,661)	(2,406)	(8,101)	511
Deferred income tax benefit (expense)	559	842	1,701	(179)
Amounts reclassified from AOCI, net of tax	(2,102)	(1,564)	(6,400)	332
Ending balance	4,657	550,776	4,657	550,776
Unrealized holding gains / losses arising on cash flow hedges
Other Comprehensive Income [Roll Forward]
Beginning balance	14,475	61,618	103,529	67,076
Change in estimate of tax reform impact			(83,806)
OCI before reclassifications	58,939	(18,245)	52,443	(25,200)
Deferred income tax benefit (expense)	(12,377)	6,386	(11,013)	8,820
AOCI before reclassification, net of tax	46,562	(11,859)	41,430	(16,380)
Amounts reclassified from AOCI	(9,059)	574	(9,206)	(868)
Deferred income tax benefit (expense)	1,902	(201)	1,933	304
Amounts reclassified from AOCI, net of tax	(7,157)	373	(7,273)	(564)
Ending balance	53,880	50,132	53,880	50,132
Unrealized holding gains / losses arising on cash flow hedges | Amount reclassified from accumulated other comprehensive income (loss)
Other Comprehensive Income [Roll Forward]
Interest expense	(887)	787	(632)	1,667
Future policy benefits, DAC and VOBA adjustments
Other Comprehensive Income [Roll Forward]
Beginning balance	(72,255)	(77,149)	(116,267)	(58,646)
Change in estimate of tax reform impact			(25,040)
OCI before reclassifications	66,954	(55,549)	154,362	(84,015)
Deferred income tax benefit (expense)	(14,060)	19,442	(32,416)	29,405
AOCI before reclassification, net of tax	52,894	(36,107)	121,946	(54,610)
Amounts reclassified from AOCI	0	0	0	0
Deferred income tax benefit (expense)	0	0	0	0
Amounts reclassified from AOCI, net of tax	0	0	0	0
Ending balance	(19,361)	(113,256)	(19,361)	(113,256)
Employee benefit plan adjustment
Other Comprehensive Income [Roll Forward]
Beginning balance	(90,734)	(82,908)	(91,192)	(84,303)
Change in estimate of tax reform impact			0
OCI before reclassifications	0	0	0	0
Deferred income tax benefit (expense)	0	0	0	0
AOCI before reclassification, net of tax	0	0	0	0
Amounts reclassified from AOCI	579	2,146	1,159	4,292
Deferred income tax benefit (expense)	(121)	(751)	(243)	(1,502)
Amounts reclassified from AOCI, net of tax	458	1,395	916	2,790
Ending balance	(90,276)	(81,513)	(90,276)	(81,513)
Total
Other Comprehensive Income [Roll Forward]
Beginning balance	137,775	299,854	440,957	235,875
Change in estimate of tax reform impact			0
OCI before reclassifications	(227,941)	163,202	(606,709)	258,010
Deferred income tax benefit (expense)	47,867	(57,121)	127,409	(90,304)
AOCI before reclassification, net of tax	(180,074)	106,081	(479,300)	167,706
Amounts reclassified from AOCI	(11,141)	314	(16,148)	3,935
Deferred income tax benefit (expense)	2,340	(110)	3,391	(1,377)
Amounts reclassified from AOCI, net of tax	(8,801)	204	(12,757)	2,558
Ending balance	$ (51,100)	$ 406,139	$ (51,100)	$ 406,139