Fair Value (Recurring Fair Value Measurements) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Assets [Abstract]
Available-for-sale Securities, Debt Securities	$ 1,863,768	$ 1,878,514
Short-term investments	1,998	0
Mortgage loans, at estimated fair value, relating to variable interest entities	407,390	406,085
Derivative assets	7,896	8,656
Separate account assets	4,876,874	4,758,449
Liabilities [Abstract]
Derivative liabilities	0	0
Embedded derivatives within liability host contracts	(36,327)	(23,740)
Recurring
Assets [Abstract]
Available-for-sale Securities, Debt Securities	1,863,768	1,878,514
Short-term investments	1,998
Embedded derivatives within asset host contracts	354,497	379,297
Separate account assets	4,876,874	4,758,449
Total assets	7,105,033	7,024,916
Liabilities [Abstract]
Embedded derivatives within liability host contracts	(36,327)	(23,740)
Total liabilities	(36,327)	(23,740)
Recurring | Foreign currency exchange rate contracts
Assets [Abstract]
Derivative assets	7,896	8,656
Recurring | U.S. corporate
Assets [Abstract]
Available-for-sale Securities, Debt Securities	732,094	721,811
Recurring | U.S. government and agency
Assets [Abstract]
Available-for-sale Securities, Debt Securities	404,888	406,545
Recurring | RMBS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	217,253	238,387
Recurring | Foreign corporate
Assets [Abstract]
Available-for-sale Securities, Debt Securities	234,366	232,340
Recurring | CMBS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	181,385	178,956
Recurring | State and political subdivision
Assets [Abstract]
Available-for-sale Securities, Debt Securities	50,709	56,320
Recurring | ABS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	26,656	26,670
Recurring | Foreign government
Assets [Abstract]
Available-for-sale Securities, Debt Securities	16,417	17,485
Recurring | Level 1
Assets [Abstract]
Available-for-sale Securities, Debt Securities	286,435	289,186
Short-term investments	0
Embedded derivatives within asset host contracts	0	0
Separate account assets	0	0
Total assets	286,435	289,186
Liabilities [Abstract]
Embedded derivatives within liability host contracts	0	0
Total liabilities	0	0
Recurring | Level 1 | Foreign currency exchange rate contracts
Assets [Abstract]
Derivative assets	0	0
Recurring | Level 1 | U.S. corporate
Assets [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Recurring | Level 1 | U.S. government and agency
Assets [Abstract]
Available-for-sale Securities, Debt Securities	286,435	289,186
Recurring | Level 1 | RMBS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Recurring | Level 1 | Foreign corporate
Assets [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Recurring | Level 1 | CMBS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Recurring | Level 1 | State and political subdivision
Assets [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Recurring | Level 1 | ABS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Recurring | Level 1 | Foreign government
Assets [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Recurring | Level 2
Assets [Abstract]
Available-for-sale Securities, Debt Securities	1,464,402	1,485,614
Short-term investments	1,998
Embedded derivatives within asset host contracts	0	0
Separate account assets	4,876,874	4,758,449
Total assets	6,351,170	6,252,719
Liabilities [Abstract]
Embedded derivatives within liability host contracts	0	0
Total liabilities	0	0
Recurring | Level 2 | Foreign currency exchange rate contracts
Assets [Abstract]
Derivative assets	7,896	8,656
Recurring | Level 2 | U.S. corporate
Assets [Abstract]
Available-for-sale Securities, Debt Securities	681,990	681,406
Recurring | Level 2 | U.S. government and agency
Assets [Abstract]
Available-for-sale Securities, Debt Securities	118,453	117,359
Recurring | Level 2 | RMBS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	197,176	217,091
Recurring | Level 2 | Foreign corporate
Assets [Abstract]
Available-for-sale Securities, Debt Securities	201,866	200,454
Recurring | Level 2 | CMBS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	176,061	173,763
Recurring | Level 2 | State and political subdivision
Assets [Abstract]
Available-for-sale Securities, Debt Securities	50,709	56,320
Recurring | Level 2 | ABS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	21,730	21,736
Recurring | Level 2 | Foreign government
Assets [Abstract]
Available-for-sale Securities, Debt Securities	16,417	17,485
Recurring | Level 3
Assets [Abstract]
Available-for-sale Securities, Debt Securities	112,931	103,714
Short-term investments	0
Embedded derivatives within asset host contracts	354,497	379,297
Separate account assets	0	0
Total assets	467,428	483,011
Liabilities [Abstract]
Embedded derivatives within liability host contracts	(36,327)	(23,740)
Total liabilities	(36,327)	(23,740)
Recurring | Level 3 | Foreign currency exchange rate contracts
Assets [Abstract]
Derivative assets	0	0
Recurring | Level 3 | U.S. corporate
Assets [Abstract]
Available-for-sale Securities, Debt Securities	50,104	40,405
Recurring | Level 3 | U.S. government and agency
Assets [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Recurring | Level 3 | RMBS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	20,077	21,296
Recurring | Level 3 | Foreign corporate
Assets [Abstract]
Available-for-sale Securities, Debt Securities	32,500	31,886
Recurring | Level 3 | CMBS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	5,324	5,193
Recurring | Level 3 | State and political subdivision
Assets [Abstract]
Available-for-sale Securities, Debt Securities	0	0
Recurring | Level 3 | ABS
Assets [Abstract]
Available-for-sale Securities, Debt Securities	4,926	4,934
Recurring | Level 3 | Foreign government
Assets [Abstract]
Available-for-sale Securities, Debt Securities	$ 0	$ 0