Derivative Financial Instruments - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Aggregate fair value of derivatives	$ 93,761,000	$ 38,324,000
Pledged collateral	42,750,000	0
Fair value of assets needed for settlement	51,011,000
Unrestricted cash collateral pledged	52,330,000	0
Collateral under derivative counterparty to satisfy netting agreements	5,490,000	103,214,000
Net derivative gains	15,415,000
Interest rate swaps
Derivative Instruments, Gain (Loss) [Line Items]
Average notional amount of the derivative instruments	905,977,000	784,900,000
Cross-currency swaps
Derivative Instruments, Gain (Loss) [Line Items]
Average notional amount of the derivative instruments	1,323,398,000	1,141,967,000
Futures
Derivative Instruments, Gain (Loss) [Line Items]
Average notional amount of the derivative instruments	108,438,000	145,658,000
Swaptions
Derivative Instruments, Gain (Loss) [Line Items]
Average notional amount of the derivative instruments	162,896,000	156,632,000
Other forward contracts
Derivative Instruments, Gain (Loss) [Line Items]
Average notional amount of the derivative instruments	2,231,196,000	2,230,167,000
Recurring basis
Derivative Instruments, Gain (Loss) [Line Items]
Collateral under derivative counterparty to satisfy netting agreements	5,490,000	103,214,000
Recurring basis | Embedded derivatives - GLWB
Derivative Instruments, Gain (Loss) [Line Items]
Fair value, measurement with unobservable inputs reconciliation, recurring basis, asset value	11,095,000	5,712,000
Recurring basis | Embedded derivatives - GLWB | Net investment income
Derivative Instruments, Gain (Loss) [Line Items]
Changes in fair value	$ 5,383,000	$ (5,545,000)