Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Securities loaned
Cash collateral received as percentage of fair value of securities loaned	102.00%
Deferred policy acquisition and sales inducement costs
Present value of future profits	$ 3,000	$ 4,000
Present value of future insurance profits, amortization expense	232	276	$ 377
Equity securities	(308,000)	(82,000)
Limited partnership interests	$ 3,147,000	2,776,000
Minimum
Deferred policy acquisition and sales inducement costs
Period for amortization of DAC and DSI for interest-sensitive life, fixed annuities and other investment contracts	15 years
Maximum
Securities loaned
Length of securities lending transaction period	30 days
Deferred policy acquisition and sales inducement costs
Period for amortization of DAC and DSI for interest-sensitive life, fixed annuities and other investment contracts	30 years
Interest-sensitive life insurance | Minimum
Deferred policy acquisition and sales inducement costs
Period for amortization of DAC and DSI for interest-sensitive life, fixed annuities and other investment contracts	10 years
Interest-sensitive life insurance | Maximum
Deferred policy acquisition and sales inducement costs
Period for amortization of DAC and DSI for interest-sensitive life, fixed annuities and other investment contracts	20 years
Fixed annuities | Minimum
Deferred policy acquisition and sales inducement costs
Period for amortization of DAC and DSI for interest-sensitive life, fixed annuities and other investment contracts	5 years
Fixed annuities | Maximum
Deferred policy acquisition and sales inducement costs
Period for amortization of DAC and DSI for interest-sensitive life, fixed annuities and other investment contracts	10 years
Accounting Standards Update 2016-01
Deferred policy acquisition and sales inducement costs
Equity securities	$ 308,000
EMA limited partnerships
Deferred policy acquisition and sales inducement costs
Limited partnership interests	2,540,000	2,190,000
EMA limited partnerships | Accounting Standards Update 2016-01
Deferred policy acquisition and sales inducement costs
Limited partnership interests	95,000
Accumulated other comprehensive income
Deferred policy acquisition and sales inducement costs
Reclassification of tax effects due to change in accounting principle	151,000	0	0
Accumulated Net Investment Gain (Loss)
Deferred policy acquisition and sales inducement costs
Reclassification of tax effects due to change in accounting principle	150,000
Accumulated Foreign Currency Adjustment
Deferred policy acquisition and sales inducement costs
Reclassification of tax effects due to change in accounting principle	1,000
Retained income
Deferred policy acquisition and sales inducement costs
Reclassification of tax effects due to change in accounting principle	$ (151,000)	$ 0	$ 0