Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Balances of Assets and Liabilities Measured at Fair Value on a Recurring Basis) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 36,345,426	$ 32,093,625
Funds withheld at interest embedded derivatives	61,281	(22,529)
Cash and cash equivalents	300,516	338,601
Short-term investments	91,024	44,241
Investments, Fair Value Disclosure	474,002	705,625
Other Assets, Fair Value Disclosure	33,349	26,958
Assets, Fair Value Disclosure, Recurring	37,305,598	33,186,521
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	974,631	990,308
Liabilities Fair Value Disclosure
Liabilities, Fair Value Disclosure, Recurring	986,456	1,011,851
Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	31,959	51,123
Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	72,318	224,238
Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	7,047	11,036
Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	55,154	93,508
Liabilities Fair Value Disclosure
Other liabilities	(17,012)	(24,374)
Foreign Currency Forwards
Liabilities Fair Value Disclosure
Other liabilities	(4,200)	(5,070)
Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(187)
Liabilities Fair Value Disclosure
Other liabilities		(262)
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	204,630	190,120
Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	6,258	9,136
Liabilities Fair Value Disclosure
Other liabilities	(319)	(5)
Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	15,804	26,070
Liabilities Fair Value Disclosure
Other liabilities	(12,497)	(7,389)
Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	81,019	100,394
Liabilities Fair Value Disclosure
Other liabilities	(1,877)	(3,231)
Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(1,552)	(2,462)
Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	22,341,661	19,619,084
Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,007,754	3,644,046
Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,539,299	1,278,576
Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,641,841	1,429,344
Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,582,028	1,363,654
US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,721,564	1,468,302
US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	638,970	591,796
Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,872,309	2,698,823
Fair Value Inputs Level 1 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,526,812	1,933,479
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	300,516	338,601
Short-term investments	21,586	8,276
Investments, Fair Value Disclosure	314,474	462,482
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	3,163,388	2,742,838
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Liabilities Fair Value Disclosure
Liabilities, Fair Value Disclosure, Recurring	0	0
Fair Value Inputs Level 1 Member | Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	31,959	38,317
Fair Value Inputs Level 1 Member | Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	72,318	224,238
Fair Value Inputs Level 1 Member | Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	7,047	11,036
Fair Value Inputs Level 1 Member | Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Foreign Currency Forwards
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0
Liabilities Fair Value Disclosure
Other liabilities		0
Fair Value Inputs Level 1 Member | Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	203,150	188,891
Fair Value Inputs Level 1 Member | Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	603,002	310,995
Fair Value Inputs Level 1 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member | US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,597,777	1,345,755
Fair Value Inputs Level 1 Member | US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 1 Member | Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	326,033	276,729
Fair Value Inputs Level 2 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	31,572,078	27,932,189
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	0	0
Short-term investments	65,890	32,619
Investments, Fair Value Disclosure	159,528	243,143
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure, Recurring	31,797,496	28,207,951
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Liabilities Fair Value Disclosure
Liabilities, Fair Value Disclosure, Recurring	10,273	19,081
Fair Value Inputs Level 2 Member | Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	12,806
Fair Value Inputs Level 2 Member | Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Fair Value Inputs Level 2 Member | Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Fair Value Inputs Level 2 Member | Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	55,154	93,508
Liabilities Fair Value Disclosure
Other liabilities	(17,012)	(24,374)
Fair Value Inputs Level 2 Member | Foreign Currency Forwards
Liabilities Fair Value Disclosure
Other liabilities	(4,200)	(5,070)
Fair Value Inputs Level 2 Member | Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(187)
Liabilities Fair Value Disclosure
Other liabilities		(262)
Fair Value Inputs Level 2 Member | Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	1,480	1,229
Fair Value Inputs Level 2 Member | Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	6,258	9,136
Liabilities Fair Value Disclosure
Other liabilities	(319)	(5)
Fair Value Inputs Level 2 Member | Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	15,804	26,070
Liabilities Fair Value Disclosure
Other liabilities	(12,497)	(7,389)
Fair Value Inputs Level 2 Member | Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	81,019	100,394
Liabilities Fair Value Disclosure
Other liabilities	(1,877)	(3,231)
Fair Value Inputs Level 2 Member | Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 2 Member | Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	20,447,605	18,035,836
Fair Value Inputs Level 2 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,474,484	3,168,081
Fair Value Inputs Level 2 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,390,614	1,118,285
Fair Value Inputs Level 2 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,440,252	1,210,064
Fair Value Inputs Level 2 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,580,085	1,342,509
Fair Value Inputs Level 2 Member | US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	100,220	98,059
Fair Value Inputs Level 2 Member | US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	604,536	550,130
Fair Value Inputs Level 2 Member | Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,534,282	2,409,225
Fair Value Inputs Level 3 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,246,536	2,227,957
Funds withheld at interest embedded derivatives	61,281	(22,529)
Cash and cash equivalents	0	0
Short-term investments	3,548	3,346
Investments, Fair Value Disclosure	0	0
Other Assets, Fair Value Disclosure	33,349	26,958
Assets, Fair Value Disclosure, Recurring	2,344,714	2,235,732
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	974,631	990,308
Liabilities Fair Value Disclosure
Liabilities, Fair Value Disclosure, Recurring	976,183	992,770
Fair Value Inputs Level 3 Member | Nonredeemable Preferred Stock [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Fair Value Inputs Level 3 Member | Other equity securities
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Fair Value Inputs Level 3 Member | Other
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Fair Value Inputs Level 3 Member | Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member | Foreign Currency Forwards
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member | Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0
Liabilities Fair Value Disclosure
Other liabilities		0
Fair Value Inputs Level 3 Member | Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Fair Value Inputs Level 3 Member | Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member | Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member | Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 3 Member | Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(1,552)	(2,462)
Fair Value Inputs Level 3 Member | Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,291,054	1,272,253
Fair Value Inputs Level 3 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	533,270	475,965
Fair Value Inputs Level 3 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	148,685	160,291
Fair Value Inputs Level 3 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	201,589	219,280
Fair Value Inputs Level 3 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,943	21,145
Fair Value Inputs Level 3 Member | US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	23,567	24,488
Fair Value Inputs Level 3 Member | US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	34,434	41,666
Fair Value Inputs Level 3 Member | Foreign Government Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 11,994	$ 12,869