Derivative Financial Instruments - Notional and Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	$ 3,131,412		$ 2,567,989
Derivative, Fair Value, Net	(103,717)		(43,184)
Asset derivatives, Fair value	34,971		51,359
Liability derivatives, Fair value	138,688		94,543
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(60,035)	$ (16,608)
Interest rate swaps \| Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(1,549)
Interest rate swaps \| Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(10,966)	0
Cross-currency swaps \| Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(14,168)
Cross-currency swaps \| Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(28,144)	0
Futures on equity indices \| Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(684)
Futures on equity indices \| Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(641)	(1,284)
Interest rate futures \| Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(14)
Interest rate futures \| Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	48	5
Interest rate swaptions \| Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(27)
Interest rate swaptions \| Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	36	(74)
Other forward contracts \| Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	6,784
Other forward contracts \| Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(20,368)	(5,597)
Cash flow hedges:
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	(6,496)	(6,955)
Gain (loss) reclassified from OCI into net income (Effective portion)	147	1,442
Cash flow hedges: \| Interest rate swaps \| Net investment income
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	(1,012)	(148)
Gain (loss) reclassified from OCI into net income (Effective portion)	928	1,220
Cash flow hedges: \| Interest rate swaps \| Interest Expense
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	29,029	3,843
Gain (loss) reclassified from OCI into net income (Effective portion)	(255)	(880)
Cash flow hedges: \| Cross-currency swaps \| Net investment income
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	(34,513)	(10,650)
Gain (loss) reclassified from OCI into net income (Effective portion)	(526)	$ 1,102
Derivatives designated as hedges:
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	886,909		1,188,860
Derivative, Fair Value, Net	(59,076)		(23,882)
Asset derivatives, Fair value	16,560		27,434
Liability derivatives, Fair value	75,636		51,316
Derivatives designated as hedges: \| Cash flow hedges:
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	886,909		1,188,860
Derivative, Fair Value, Net	(59,076)		(23,882)
Asset derivatives, Fair value	16,560		27,434
Liability derivatives, Fair value	75,636		51,316
Derivatives designated as hedges: \| Cash flow hedges: \| Interest rate swaps
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	22,300		388,800
Derivative, Fair Value, Net	6,156		7,476
Asset derivatives, Fair value	6,156		7,476
Liability derivatives, Fair value	0		0
Derivatives designated as hedges: \| Cash flow hedges: \| Cross-currency swaps
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	864,609		800,060
Derivative, Fair Value, Net	(65,232)		(31,358)
Asset derivatives, Fair value	10,404		19,958
Liability derivatives, Fair value	75,636		51,316
Derivatives not designated as hedges:
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	2,244,503		1,379,129
Derivative, Fair Value, Net	(44,641)		(19,302)
Asset derivatives, Fair value	18,411		23,925
Liability derivatives, Fair value	63,052		43,227
Derivatives not designated as hedges: \| Interest rate swaps
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	534,100		519,100
Derivative, Fair Value, Net	775		1,902
Asset derivatives, Fair value	1,703		3,530
Liability derivatives, Fair value	928		1,628
Derivatives not designated as hedges: \| Cross-currency swaps
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	612,733		612,733
Derivative, Fair Value, Net	(49,242)		(21,279)
Asset derivatives, Fair value	12,882		20,320
Liability derivatives, Fair value	62,124		41,599
Derivatives not designated as hedges: \| Futures on equity indices
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	57,609		22,074
Derivative, Fair Value, Net	0		0
Asset derivatives, Fair value	0		0
Liability derivatives, Fair value	0		0
Derivatives not designated as hedges: \| Interest rate futures
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	47,300		60,700
Derivative, Fair Value, Net	0		0
Asset derivatives, Fair value	0		0
Liability derivatives, Fair value	0		0
Derivatives not designated as hedges: \| Interest rate swaptions
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	172,761		164,522
Derivative, Fair Value, Net	135		75
Asset derivatives, Fair value	135		75
Liability derivatives, Fair value	0		$ 0
Derivatives not designated as hedges: \| Other forward contracts
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Derivative, Notional Amount	820,000
Derivative, Fair Value, Net	3,691
Asset derivatives, Fair value	3,691
Liability derivatives, Fair value	$ 0

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

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The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

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Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

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