Derivative Financial Instruments - Notional and Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	$ 4,749,878		$ 4,749,878		$ 2,567,989
Net derivatives, Fair value	(10,681)		(10,681)		(43,184)
Asset derivatives, Fair value	62,869		62,869		51,359
Liability derivatives, Fair value	73,550		73,550		94,543
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	34,204	$ 4,889	(25,831)	$ (11,720)
Interest rate swaps | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	4,976	0	3,427
Interest rate swaps | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(3,336)	0	(14,302)	0
Cross-currency swaps | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(8,083)	0	(22,251)
Cross-currency swaps | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	40,739	0	12,595	0
Futures on equity indices | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	350	0	(334)
Futures on equity indices | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(1,504)	(1,536)	(2,145)	(2,820)
Interest rate futures | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	116	0	102
Interest rate futures | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	67	(206)	115	(201)
Interest rate swaptions | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(4)	0	(31)
Interest rate swaptions | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(187)	(77)	(151)	(151)
Other forward contracts | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(9,363)	0	(2,580)
Other forward contracts | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(1,575)	18,716	(21,943)	13,119
Cash flow hedges:
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	58,939	(18,245)	52,443	(25,200)
Gain (loss) reclassified from OCI into net income (Effective portion)	9,059	(574)	9,206	868
Cash flow hedges: | Interest rate swaps | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	(319)	644	(1,331)	496
Gain (loss) reclassified from OCI into net income (Effective portion)	705	1,186	1,633	2,406
Cash flow hedges: | Interest rate swaps | Interest Expense
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	1	(8,193)	29,030	(4,350)
Gain (loss) reclassified from OCI into net income (Effective portion)	887	(787)	632	(1,667)
Cash flow hedges: | Cross-currency swaps | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	59,257	(10,696)	24,744	(21,346)
Gain (loss) reclassified from OCI into net income (Effective portion)	7,467	$ (973)	6,941	$ 129
Derivatives designated as hedges:
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	886,454		886,454		1,188,860
Net derivatives, Fair value	(2,937)		(2,937)		(23,882)
Asset derivatives, Fair value	36,388		36,388		27,434
Liability derivatives, Fair value	39,325		39,325		51,316
Derivatives designated as hedges: | Cash flow hedges:
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	886,454		886,454		1,188,860
Net derivatives, Fair value	(2,937)		(2,937)		(23,882)
Asset derivatives, Fair value	36,388		36,388		27,434
Liability derivatives, Fair value	39,325		39,325		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
Notional amount	22,300		22,300		388,800
Net derivatives, Fair value	5,917		5,917		7,476
Asset derivatives, Fair value	5,917		5,917		7,476
Liability derivatives, Fair value	0		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
Notional amount	864,154		864,154		800,060
Net derivatives, Fair value	(8,854)		(8,854)		(31,358)
Asset derivatives, Fair value	30,471		30,471		19,958
Liability derivatives, Fair value	39,325		39,325		51,316
Derivatives not designated as hedges:
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	3,863,424		3,863,424		1,379,129
Net derivatives, Fair value	(7,744)		(7,744)		(19,302)
Asset derivatives, Fair value	26,481		26,481		23,925
Liability derivatives, Fair value	34,225		34,225		43,227
Derivatives not designated as hedges: | Interest rate swaps
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	548,500		548,500		519,100
Net derivatives, Fair value	577		577		1,902
Asset derivatives, Fair value	1,461		1,461		3,530
Liability derivatives, Fair value	884		884		1,628
Derivatives not designated as hedges: | Cross-currency swaps
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	573,703		573,703		612,733
Net derivatives, Fair value	(11,778)		(11,778)		(21,279)
Asset derivatives, Fair value	20,292		20,292		20,320
Liability derivatives, Fair value	32,070		32,070		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
Notional amount	56,521		56,521		22,074
Net derivatives, Fair value	0		0		0
Asset derivatives, Fair value	0		0		0
Liability derivatives, Fair value	0		0		0
Derivatives not designated as hedges: | Interest rate futures
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	34,300		34,300		60,700
Net derivatives, Fair value	0		0		0
Asset derivatives, Fair value	0		0		0
Liability derivatives, Fair value	0		0		0
Derivatives not designated as hedges: | Interest rate swaptions
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	190,400		190,400		164,522
Net derivatives, Fair value	163		163		75
Asset derivatives, Fair value	163		163		75
Liability derivatives, Fair value	0		0		$ 0
Derivatives not designated as hedges: | Other forward contracts
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	2,460,000		2,460,000
Net derivatives, Fair value	3,294		3,294
Asset derivatives, Fair value	4,565		4,565
Liability derivatives, Fair value	$ 1,271		$ 1,271