Equity (Reclassification out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Total net unrealized investment gains (losses)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI	$ 26,998	$ 64,540	$ 9,902
Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI	26,998	64,540	9,902
Reclassification out of Accumulated Other Comprehensive Income | Cash flow hedges—Currency/Interest rate
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized Gain (Loss) on Investments	(1,103)	7,915	4,286
Reclassification out of Accumulated Other Comprehensive Income | Net unrealized investment gains (losses) on available-for-sale securities
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized Gain (Loss) on Investments	28,101	56,625	5,616
Reclassification out of Accumulated Other Comprehensive Income | Total net unrealized investment gains (losses)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI	26,998	64,540	9,902
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Other Comprehensive Income
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from AOCI	$ 26,998	$ 64,540	$ 9,902