Other Comprehensive Income - OCI - Changes in Accumulated Other comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Comprehensive Income [Roll Forward]
Beginning balance			$ 2,449,589	$ 2,012,321
Deferred income tax benefit (expense)	$ (17,287)	$ (21,688)	(58,091)	(64,973)
Ending balance	1,991,163	2,260,897	1,991,163	2,260,897
Interest expense	(7,082)	(7,811)	(24,479)	(23,087)
Unrealized holding gains / losses arising on fixed maturities, available-for- sale
Other Comprehensive Income [Roll Forward]
Beginning balance	4,657	550,776	544,887	311,748
Change in estimate of tax reform impact			108,846
OCI before reclassifications	(148,340)	39,983	(961,854)	407,208
Deferred income tax benefit (expense)	31,151	(13,994)	201,989	(142,523)
AOCI before reclassification, net of tax	(117,189)	25,989	(759,865)	264,685
Amounts reclassified from AOCI	(2,219)	(2,508)	(10,320)	(1,997)
Deferred income tax benefit (expense)	467	878	2,168	699
Amounts reclassified from AOCI, net of tax	(1,752)	(1,630)	(8,152)	(1,298)
Ending balance	(114,284)	575,135	(114,284)	575,135
Unrealized holding gains / losses arising on cash flow hedges
Other Comprehensive Income [Roll Forward]
Beginning balance	53,880	50,132	103,529	67,076
Change in estimate of tax reform impact			(83,806)
OCI before reclassifications	4,063	(23,111)	56,506	(48,311)
Deferred income tax benefit (expense)	(853)	8,089	(11,866)	16,909
AOCI before reclassification, net of tax	3,210	(15,022)	44,640	(31,402)
Amounts reclassified from AOCI	(4,718)	(377)	(13,924)	(1,245)
Deferred income tax benefit (expense)	991	132	2,924	436
Amounts reclassified from AOCI, net of tax	(3,727)	(245)	(11,000)	(809)
Ending balance	53,363	34,865	53,363	34,865
Unrealized holding gains / losses arising on cash flow hedges | Amount reclassified from accumulated other comprehensive income (loss)
Other Comprehensive Income [Roll Forward]
Interest expense	(443)	690	(1,075)	2,357
Future policy benefits, DAC and VOBA adjustments
Other Comprehensive Income [Roll Forward]
Beginning balance	(19,361)	(113,256)	(116,267)	(58,646)
Change in estimate of tax reform impact			(25,040)
OCI before reclassifications	35,053	(1,977)	189,415	(85,992)
Deferred income tax benefit (expense)	(7,361)	692	(39,777)	30,097
AOCI before reclassification, net of tax	27,692	(1,285)	149,638	(55,895)
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	8,331	(114,541)	8,331	(114,541)
Employee benefit plan adjustment
Other Comprehensive Income [Roll Forward]
Beginning balance	(90,276)	(81,513)	(91,192)	(84,303)
Change in estimate of tax reform impact			0
OCI before reclassifications	0	8,553	0	8,553
Deferred income tax benefit (expense)	0	(2,994)	0	(2,994)
AOCI before reclassification, net of tax	0	5,559	0	5,559
Amounts reclassified from AOCI	646	2,193	1,805	6,485
Deferred income tax benefit (expense)	(136)	(768)	(379)	(2,270)
Amounts reclassified from AOCI, net of tax	510	1,425	1,426	4,215
Ending balance	(89,766)	(74,529)	(89,766)	(74,529)
Total
Other Comprehensive Income [Roll Forward]
Beginning balance	(51,100)	406,139	440,957	235,875
Change in estimate of tax reform impact			0
OCI before reclassifications	(109,224)	23,448	(715,933)	281,458
Deferred income tax benefit (expense)	22,937	(8,207)	150,346	(98,511)
AOCI before reclassification, net of tax	(86,287)	15,241	(565,587)	182,947
Amounts reclassified from AOCI	(6,291)	(692)	(22,439)	3,243
Deferred income tax benefit (expense)	1,322	242	4,713	(1,135)
Amounts reclassified from AOCI, net of tax	(4,969)	(450)	(17,726)	2,108
Ending balance	$ (142,356)	$ 420,930	$ (142,356)	$ 420,930