Certain Long-Duration Contracts with Guarantees (Variable Annuity Contracts) (Details) - Annuity Contracts - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Return of net deposits | In the Event of Death(2)
Net Amount at Risk by Product and Guarantee [Line Items]
Account value	$ 119,182,143	$ 110,194,439
Net amount at risk	$ 274,617	$ 463,423
Average attained age of contractholders	66 years	66 years
Minimum return or contract value | In the Event of Death(2)
Net Amount at Risk by Product and Guarantee [Line Items]
Account value	$ 25,835,100	$ 24,725,084
Net amount at risk	$ 2,161,133	$ 3,098,018
Average attained age of contractholders	69 years	69 years
Minimum return or contract value | At Annuitization / Accumulation
Net Amount at Risk by Product and Guarantee [Line Items]
Account value	$ 129,630,456	$ 120,237,955
Net amount at risk	$ 3,225,700	$ 5,041,214
Average attained age of contractholders	67 years	66 years
Average period remaining until earliest expected annuitization	0 years	0 years