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	Sep. 30, 2018	Dec. 31, 2017
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 39,005,212	$ 38,150,820
Non Redeemable Preferred Stock Equity Method Investments Fair Value Disclosure		39,806
Equity Method Investments, Fair Value Disclosure	101,069	60,346
Funds withheld at interest embedded derivatives	142,529	122,194
Cash and cash equivalents	657,280	356,788
Short-term investments	186,051	50,746
Investments, Fair Value Disclosure	313,439	356,154
Other Assets, Fair Value Disclosure	46,090	40,659
Assets, Fair Value Disclosure	40,451,670	39,177,513
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	906,199	1,014,228
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	920,131	1,038,469
Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	29,106	51,359
Liabilities Fair Value Disclosure
Other liabilities	(14,166)	(14,311)
Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	6,881	730
Liabilities Fair Value Disclosure
Other liabilities		(7,213)
Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(13)	(221)
Liabilities Fair Value Disclosure
Other liabilities	(2,696)	(1,939)
Fair Value Option Contractholder Directed Unit Linked Investments [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	212,315	218,541
Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	5,961	5,908
Liabilities Fair Value Disclosure
Other liabilities	(684)	(760)
Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	16,818	16,932
Liabilities Fair Value Disclosure
Other liabilities	(2,554)	(6,339)
Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	42,371	62,905
Liabilities Fair Value Disclosure
Other liabilities		(6,194)
Mortality Swap [Member]
Liabilities Fair Value Disclosure
Other liabilities	(308)	(1,683)
Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	24,449,498	23,210,968
Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	4,036,750	4,220,076
Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,780,762	1,719,880
Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,863,017	1,648,362
Assets, Fair Value Disclosure	64,475	0
Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,273,534	1,303,387
US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,412,409	1,943,592
US States and Political Subdivisions Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	911,991	703,428
Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,277,251	3,401,127
Fair Value Inputs Level 1 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,284,932	1,818,006
Non Redeemable Preferred Stock Equity Method Investments Fair Value Disclosure		39,806
Equity Method Investments, Fair Value Disclosure	57,584	60,346
Funds withheld at interest embedded derivatives	0	0
Cash and cash equivalents	657,280	354,071
Short-term investments	5,957	0
Investments, Fair Value Disclosure	211,253	217,618
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	2,217,006	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
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	211,253	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	1,284,932	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	35,792,145	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	0	2,717
Short-term investments	177,891	47,650
Investments, Fair Value Disclosure	102,186	138,536
Other Assets, Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	36,072,222	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	13,624	22,558
Fair Value Inputs Level 2 Member | Interest Rate Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	29,106	51,359
Liabilities Fair Value Disclosure
Other liabilities	(14,166)	(14,311)
Fair Value Inputs Level 2 Member | Foreign Currency Forwards
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	6,881	730
Liabilities Fair Value Disclosure
Other liabilities		(7,213)
Fair Value Inputs Level 2 Member | Consumer Price Index Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	(13)	(221)
Liabilities Fair Value Disclosure
Other liabilities	(2,696)	(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	1,062	923
Fair Value Inputs Level 2 Member | Credit Default Swaps
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	5,961	5,908
Liabilities Fair Value Disclosure
Other liabilities	(684)	(760)
Fair Value Inputs Level 2 Member | Equity Option [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	16,818	16,932
Liabilities Fair Value Disclosure
Other liabilities	(2,554)	(6,339)
Fair Value Inputs Level 2 Member | Currency Swap [Member]
Assets Fair Value Disclosure
Investments, Fair Value Disclosure	42,371	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	23,193,563	21,873,696
Fair Value Inputs Level 2 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,488,008	3,626,134
Fair Value Inputs Level 2 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,780,762	1,611,998
Fair Value Inputs Level 2 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,780,250	1,524,888
Fair Value Inputs Level 2 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,271,752	1,300,153
Fair Value Inputs Level 2 Member | US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	107,225	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	898,346	662,225
Fair Value Inputs Level 2 Member | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	3,272,239	3,396,035
Fair Value Inputs Level 3 Member
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,928,135	2,234,610
Non Redeemable Preferred Stock Equity Method Investments Fair Value Disclosure		0
Equity Method Investments, Fair Value Disclosure	43,485	0
Funds withheld at interest embedded derivatives	142,529	122,194
Cash and cash equivalents	0	0
Short-term investments	2,203	3,096
Investments, Fair Value Disclosure	0	0
Other Assets, Fair Value Disclosure	46,090	40,659
Assets, Fair Value Disclosure	2,162,442	2,400,559
Fair Value Measured On Recurring Basis Guaranteed Interest Contracts Embedded Derivatives	906,199	1,014,228
Liabilities Fair Value Disclosure
Financial Liabilities Fair Value Disclosure	906,507	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
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	(308)	(1,683)
Fair Value Inputs Level 3 Member | Corporate Debt Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,255,935	1,337,272
Fair Value Inputs Level 3 Member | Canadian and Canadian provincial governments
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	548,742	593,942
Fair Value Inputs Level 3 Member | Residential mortgage-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	0	107,882
Fair Value Inputs Level 3 Member | Asset-backed securities
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	82,767	123,474
Fair Value Inputs Level 3 Member | Commercial Mortgage Backed Securities [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	1,782	3,234
Fair Value Inputs Level 3 Member | US Treasury and Government [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	20,252	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	13,645	41,203
Fair Value Inputs Level 3 Member | Debt Security, Government, Non-US [Member]
Assets Fair Value Disclosure
Fixed maturity securities, available-for-sale	$ 5,012	$ 5,092