Derivative Financial Instruments - Notional and Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	$ 4,380,835		$ 4,380,835		$ 2,567,989
Net derivatives, Fair value	(13,066)		(13,066)		(43,184)
Asset derivatives, Fair value	60,318		60,318		51,359
Liability derivatives, Fair value	73,384		73,384		94,543
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(8,192)	$ (10,615)	(34,023)	$ (22,335)
Interest rate swaps | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	492	0	3,919
Interest rate swaps | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(6,651)	0	(20,953)	0
Cross-currency swaps | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(16,046)	0	(38,297)
Cross-currency swaps | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	5,570	0	18,165	0
Futures on equity indices | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(583)	0	(917)
Futures on equity indices | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(2,511)	(1,070)	(4,656)	(3,890)
Interest rate futures | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(40)	0	62
Interest rate futures | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(146)	18	(31)	(183)
Interest rate swaptions | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(6)	0	(37)
Interest rate swaptions | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	1,003	(73)	852	(224)
Other forward contracts | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	0	(571)	0	(3,151)
Other forward contracts | Realized investment gains (losses)
Derivatives not designated as hedging instruments
Gain (loss) on derivatives recognized in net income	(5,457)	7,264	(27,400)	20,383
Cash flow hedges:
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	4,063	(23,111)	56,506	(48,311)
Gain (loss) reclassified from OCI into net income (Effective portion)	4,718	377	13,924	1,245
Cash flow hedges: | Interest rate swaps | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	(596)	115	(1,927)	611
Gain (loss) reclassified from OCI into net income (Effective portion)	579	1,175	2,212	3,581
Cash flow hedges: | Interest rate swaps | Interest Expense
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	0	(653)	29,030	(5,003)
Gain (loss) reclassified from OCI into net income (Effective portion)	443	(690)	1,075	(2,357)
Cash flow hedges: | Cross-currency swaps | Net investment income
Derivatives not designated as hedging instruments
Gain (loss) recognized in OCI on derivatives (Effective portion)	4,659	(22,573)	29,403	(43,919)
Gain (loss) reclassified from OCI into net income (Effective portion)	3,696	$ (108)	10,637	$ 21
Derivatives designated as hedges:
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	908,318		908,318		1,188,860
Net derivatives, Fair value	(1,480)		(1,480)		(23,882)
Asset derivatives, Fair value	36,062		36,062		27,434
Liability derivatives, Fair value	37,542		37,542		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	908,318		908,318		1,188,860
Net derivatives, Fair value	(1,480)		(1,480)		(23,882)
Asset derivatives, Fair value	36,062		36,062		27,434
Liability derivatives, Fair value	37,542		37,542		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	5,037		5,037		7,476
Asset derivatives, Fair value	5,037		5,037		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	886,018		886,018		800,060
Net derivatives, Fair value	(6,517)		(6,517)		(31,358)
Asset derivatives, Fair value	31,025		31,025		19,958
Liability derivatives, Fair value	37,542		37,542		51,316
Derivatives not designated as hedges:
Effect of derivative instruments in the condensed consolidated statement of income and comprehensive income
Notional amount	3,472,517		3,472,517		1,379,129
Net derivatives, Fair value	(11,586)		(11,586)		(19,302)
Asset derivatives, Fair value	24,256		24,256		23,925
Liability derivatives, Fair value	35,842		35,842		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	560,500		560,500		519,100
Net derivatives, Fair value	(262)		(262)		1,902
Asset derivatives, Fair value	681		681		3,530
Liability derivatives, Fair value	943		943		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	(6,404)		(6,404)		(21,279)
Asset derivatives, Fair value	22,435		22,435		20,320
Liability derivatives, Fair value	28,839		28,839		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	58,344		58,344		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	33,300		33,300		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	192,670		192,670		164,522
Net derivatives, Fair value	164		164		75
Asset derivatives, Fair value	164		164		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	2,054,000		2,054,000
Net derivatives, Fair value	(5,084)		(5,084)
Asset derivatives, Fair value	976		976
Liability derivatives, Fair value	$ 6,060		$ 6,060