Other Comprehensive Income - OCI - Changes in Accumulated Other comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other Comprehensive Income [Roll Forward]
Beginning balance	$ 2,449,589	$ 2,012,321
Deferred income tax benefit (expense)	(13,553)	(17,117)
Ending balance	2,205,082	2,034,550
Interest expense	7,809	7,630
Unrealized holding gains / losses arising on fixed maturities, available-for- sale
Other Comprehensive Income [Roll Forward]
Beginning balance	544,887	311,748
Change in estimate of tax reform impact	108,846
OCI before reclassifications	(459,680)	130,229
Deferred income tax benefit (expense)	96,534	(45,580)
AOCI before reclassification, net of tax	(363,146)	84,649
Amounts reclassified from AOCI	(5,440)	2,917
Deferred income tax benefit (expense)	1,142	(1,021)
Amounts reclassified from AOCI, net of tax	(4,298)	1,896
Ending balance	286,289	398,293
Unrealized holding gains / losses arising on cash flow hedges
Other Comprehensive Income [Roll Forward]
Beginning balance	103,529	67,076
Change in estimate of tax reform impact	(83,806)
OCI before reclassifications	(6,496)	(6,955)
Deferred income tax benefit (expense)	1,364	2,434
AOCI before reclassification, net of tax	(5,132)	(4,521)
Amounts reclassified from AOCI	(147)	(1,442)
Deferred income tax benefit (expense)	31	505
Amounts reclassified from AOCI, net of tax	(116)	(937)
Ending balance	14,475	61,618
Unrealized holding gains / losses arising on cash flow hedges | Amount reclassified from accumulated other comprehensive income (loss)
Other Comprehensive Income [Roll Forward]
Interest expense	(255)	(880)
Future policy benefits, DAC and VOBA adjustments
Other Comprehensive Income [Roll Forward]
Beginning balance	(116,267)	(58,646)
Change in estimate of tax reform impact	(25,040)
OCI before reclassifications	87,408	(28,466)
Deferred income tax benefit (expense)	(18,356)	9,963
AOCI before reclassification, net of tax	69,052	(18,503)
Amounts reclassified from AOCI	0	0
Deferred income tax benefit (expense)	0	0
Amounts reclassified from AOCI, net of tax	0	0
Ending balance	(72,255)	(77,149)
Employee benefit plan adjustment
Other Comprehensive Income [Roll Forward]
Beginning balance	(91,192)	(84,303)
Change in estimate of tax reform impact	0
OCI before reclassifications	0	0
Deferred income tax benefit (expense)	0	0
AOCI before reclassification, net of tax	0	0
Amounts reclassified from AOCI	580	2,146
Deferred income tax benefit (expense)	(122)	(751)
Amounts reclassified from AOCI, net of tax	458	1,395
Ending balance	(90,734)	(82,908)
Total
Other Comprehensive Income [Roll Forward]
Beginning balance	440,957	235,875
Change in estimate of tax reform impact	0
OCI before reclassifications	(378,768)	94,808
Deferred income tax benefit (expense)	79,542	(33,183)
AOCI before reclassification, net of tax	(299,226)	61,625
Amounts reclassified from AOCI	(5,007)	3,621
Deferred income tax benefit (expense)	1,051	(1,267)
Amounts reclassified from AOCI, net of tax	(3,956)	2,354
Ending balance	$ 137,775	$ 299,854