Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	$ 1,951,892	$ 1,713,065
Liability for future policy benefits, losses, claims and loss expense reserves	5,954,524	5,673,890
Unearned premiums	486	527
Other policyholders' funds	307,401	268,136
Separate account liabilities	2,572,872	2,287,953
Premium revenue	961,338	843,709	$ 750,224
Net investment income	79,017	79,025	76,509
Benefits, claims, losses and settlement expense	416,019	367,655	339,315
Amortization of deferred policy acquisition costs	209,399	180,582	157,727
Other operating expense	684,164	633,252	616,179
Premiums written	821	844	908
Term Life Insurance [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	1,861,253	1,628,957
Liability for future policy benefits, losses, claims and loss expense reserves	5,747,317	5,464,851
Other policyholders' funds	299,265	258,774
Premium revenue	941,057	822,207	728,181
Net investment income	9,931	7,634	5,985
Benefits, claims, losses and settlement expense	398,212	350,640	322,232
Amortization of deferred policy acquisition costs	201,751	172,812	147,980
Other operating expense	146,604	129,569	120,538
Investment and Savings Products [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	64,419	56,933
Separate account liabilities	2,572,766	2,287,829
Amortization of deferred policy acquisition costs	6,168	6,148	7,951
Other operating expense	403,743	374,117	367,301
Corporate and Other Distributed Products Segment [Member]
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	26,220	27,175
Liability for future policy benefits, losses, claims and loss expense reserves	207,207	209,039
Unearned premiums	486	527
Other policyholders' funds	8,136	9,362
Separate account liabilities	106	124
Premium revenue	20,281	21,502	22,043
Net investment income	69,086	71,391	70,524
Benefits, claims, losses and settlement expense	17,807	17,015	17,083
Amortization of deferred policy acquisition costs	1,480	1,622	1,796
Other operating expense	133,817	129,566	128,340
Premiums written	$ 821	$ 844	$ 908