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, 2018	Dec. 31, 2017
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 36,784,954	$ 38,150,820
Non Redeemable Preferred Stock Equity Method Investments Fair Value Disclosure		39,806
Equity Method Investments, Fair Value Disclosure	108,070	60,346
Funds withheld at interest embedded derivatives	144,610	122,194
Cash and cash equivalents	424,601	356,788
Short-term investments	82,521	50,746
Investments, Fair Value Disclosure	349,517	356,154
Other Assets, Fair Value Disclosure	43,971	40,659
Assets, Fair Value Disclosure	37,938,244	39,177,513
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	928,797	1,014,228
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	937,868	1,038,469
Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	36,170	51,359
Liabilities Fair Value Disclosure
Other liabilities	(14,740)	(14,311)
Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	18,428	730
Liabilities Fair Value Disclosure
Other liabilities	(8)	(7,213)
Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(42)	(221)
Liabilities Fair Value Disclosure
Other liabilities	969	(1,939)
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	212,202	218,541
Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	5,927	5,908
Liabilities Fair Value Disclosure
Other liabilities	(676)	(760)
Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	21,136	16,932
Liabilities Fair Value Disclosure
Other liabilities	(4,814)	(6,339)
Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	55,696	62,905
Liabilities Fair Value Disclosure
Other liabilities		(6,194)
Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(782)	(1,683)
Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	22,503,507	23,210,968
Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,062,843	4,220,076
Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,821,214	1,719,880
Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,708,824	1,648,362
Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,242,509	1,303,387
US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,525,150	1,943,592
US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	737,044	703,428
Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,183,863	3,401,127
Fair Value Inputs Level 1 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,405,485	1,818,006
Non Redeemable Preferred Stock Equity Method Investments Fair Value Disclosure		39,806
Equity Method Investments, Fair Value Disclosure	65,133	60,346
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	409,242	354,071
Short-term investments	991	0
Investments, Fair Value Disclosure	211,141	217,618
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	2,091,992	2,489,847
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	0	0
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
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
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	0
Liabilities Fair Value Disclosure
Other liabilities	0	0
Fair Value Inputs Level 1 Member | Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	211,141	217,618
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
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	0	0
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	1,405,485	1,818,006
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 | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	0
Fair Value Inputs Level 2 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	33,269,941	34,098,204
Non Redeemable Preferred Stock Equity Method Investments Fair Value Disclosure		0
Equity Method Investments, Fair Value Disclosure	0	0
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	15,359	2,717
Short-term investments	78,313	47,650
Investments, Fair Value Disclosure	138,376	138,536
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	33,501,989	34,287,107
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	0	0
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	8,289	22,558
Fair Value Inputs Level 2 Member | Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	36,170	51,359
Liabilities Fair Value Disclosure
Other liabilities	(14,740)	(14,311)
Fair Value Inputs Level 2 Member | Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	18,428	730
Liabilities Fair Value Disclosure
Other liabilities	(8)	(7,213)
Fair Value Inputs Level 2 Member | Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(42)	(221)
Liabilities Fair Value Disclosure
Other liabilities	969	(1,939)
Fair Value Inputs Level 2 Member | Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	1,061	923
Fair Value Inputs Level 2 Member | Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	5,927	5,908
Liabilities Fair Value Disclosure
Other liabilities	(676)	(760)
Fair Value Inputs Level 2 Member | Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	21,136	16,932
Liabilities Fair Value Disclosure
Other liabilities	(4,814)	(6,339)
Fair Value Inputs Level 2 Member | Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	55,696	62,905
Liabilities Fair Value Disclosure
Other liabilities		(6,194)
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	21,136,353	21,873,696
Fair Value Inputs Level 2 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,490,145	3,626,134
Fair Value Inputs Level 2 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,766,375	1,611,998
Fair Value Inputs Level 2 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,638,138	1,524,888
Fair Value Inputs Level 2 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,240,642	1,300,153
Fair Value Inputs Level 2 Member | US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	98,930	103,075
Fair Value Inputs Level 2 Member | US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	720,539	662,225
Fair Value Inputs Level 2 Member | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,178,819	3,396,035
Fair Value Inputs Level 3 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	2,109,528	2,234,610
Non Redeemable Preferred Stock Equity Method Investments Fair Value Disclosure		0
Equity Method Investments, Fair Value Disclosure	42,937	0
Funds withheld at interest embedded derivatives	144,610	122,194
Cash and cash equivalents	0	0
Short-term investments	3,217	3,096
Investments, Fair Value Disclosure	0	0
Other Assets, Fair Value Disclosure	43,971	40,659
Assets, Fair Value Disclosure	2,344,263	2,400,559
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	928,797	1,014,228
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	929,579	1,015,911
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
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	0	0
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	0
Liabilities Fair Value Disclosure
Other liabilities	0	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
Fair Value Inputs Level 3 Member | Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(782)	(1,683)
Fair Value Inputs Level 3 Member | Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,367,154	1,337,272
Fair Value Inputs Level 3 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	572,698	593,942
Fair Value Inputs Level 3 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	54,839	107,882
Fair Value Inputs Level 3 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	70,686	123,474
Fair Value Inputs Level 3 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,867	3,234
Fair Value Inputs Level 3 Member | US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	20,735	22,511
Fair Value Inputs Level 3 Member | US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	16,505	41,203
Fair Value Inputs Level 3 Member | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 5,044	$ 5,092