Derivative Instruments (Financial Statement Classification and Impact of Derivatives Used in Qualifying and Non-qualifying Hedge Relationships) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	$ (106,625)	$ 664,533	$ (220,292)
Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	6,071	4,055	3,297
Other Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(8,305)	1,623	2,088
AOCI
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(58,609)	(7,340)	36,686
Derivatives Designated as Hedge Accounting Instruments: | Cash flow hedges | Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Derivatives Designated as Hedge Accounting Instruments: | Cash flow hedges | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	6,071	4,055	3,297
Derivatives Designated as Hedge Accounting Instruments: | Cash flow hedges | Other Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(8,234)	1,638	1,879
Derivatives Designated as Hedge Accounting Instruments: | Cash flow hedges | AOCI
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(58,609)	(7,340)	36,686
Derivatives Not Qualifying as Hedge Accounting Instruments: | Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(106,625)	664,533	(220,292)
Derivatives Not Qualifying as Hedge Accounting Instruments: | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Derivatives Not Qualifying as Hedge Accounting Instruments: | Other Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(71)	(15)	209
Derivatives Not Qualifying as Hedge Accounting Instruments: | AOCI
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Interest Rate | Derivatives Not Qualifying as Hedge Accounting Instruments: | Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(1,565)	186,419	77,158
Interest Rate | Derivatives Not Qualifying as Hedge Accounting Instruments: | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Interest Rate | Derivatives Not Qualifying as Hedge Accounting Instruments: | Other Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Interest Rate | Derivatives Not Qualifying as Hedge Accounting Instruments: | AOCI
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Currency | Derivatives Not Qualifying as Hedge Accounting Instruments: | Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(1,316)	1,657	211
Currency | Derivatives Not Qualifying as Hedge Accounting Instruments: | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Currency | Derivatives Not Qualifying as Hedge Accounting Instruments: | Other Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Currency | Derivatives Not Qualifying as Hedge Accounting Instruments: | AOCI
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Currency/Interest Rate | Derivatives Designated as Hedge Accounting Instruments: | Cash flow hedges | Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Currency/Interest Rate | Derivatives Designated as Hedge Accounting Instruments: | Cash flow hedges | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	6,071	4,055	3,297
Currency/Interest Rate | Derivatives Designated as Hedge Accounting Instruments: | Cash flow hedges | Other Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(8,234)	1,638	1,879
Currency/Interest Rate | Derivatives Designated as Hedge Accounting Instruments: | Cash flow hedges | AOCI
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(58,609)	(7,340)	36,686
Currency/Interest Rate | Derivatives Not Qualifying as Hedge Accounting Instruments: | Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(7,245)	8,960	11,533
Currency/Interest Rate | Derivatives Not Qualifying as Hedge Accounting Instruments: | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Currency/Interest Rate | Derivatives Not Qualifying as Hedge Accounting Instruments: | Other Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(71)	(15)	209
Currency/Interest Rate | Derivatives Not Qualifying as Hedge Accounting Instruments: | AOCI
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Credit | Derivatives Not Qualifying as Hedge Accounting Instruments: | Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(46)	(535)	90
Credit | Derivatives Not Qualifying as Hedge Accounting Instruments: | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Credit | Derivatives Not Qualifying as Hedge Accounting Instruments: | Other Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Credit | Derivatives Not Qualifying as Hedge Accounting Instruments: | AOCI
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Equity | Derivatives Not Qualifying as Hedge Accounting Instruments: | Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	30,466	350	(35,276)
Equity | Derivatives Not Qualifying as Hedge Accounting Instruments: | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Equity | Derivatives Not Qualifying as Hedge Accounting Instruments: | Other Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Equity | Derivatives Not Qualifying as Hedge Accounting Instruments: | AOCI
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Embedded Derivatives | Derivatives Not Qualifying as Hedge Accounting Instruments: | Realized Investment Gains (Losses)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	(126,919)	467,682	(274,008)
Embedded Derivatives | Derivatives Not Qualifying as Hedge Accounting Instruments: | Net Investment Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Embedded Derivatives | Derivatives Not Qualifying as Hedge Accounting Instruments: | Other Income
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	0	0	0
Embedded Derivatives | Derivatives Not Qualifying as Hedge Accounting Instruments: | AOCI
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, gain (loss) on derivative, net	$ 0	$ 0	$ 0