Investments Investments (Narrative) (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2017 USD ($) position	Dec. 31, 2016 USD ($) position	Dec. 31, 2015 USD ($)
Investment Narrative [Line Items]
Securities Sold under Agreements to Repurchase, Collateral, Right to Reclaim Cash		$ 31,200	$ 28,800
Net Amount Assets Liabilities From Repurchase Reverse Repurchase Agreements		1,100	5,500
Available-for-sale Securities, Amortized Cost Basis		21,966,803	18,924,711
Available-for-sale Securities		$ 23,210,968	$ 19,619,084
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | position		1,116	1,535
Proceeds From Sale Of Available For Sale Securities Sold At Loss		$ 2,727,800	$ 1,181,600	$ 1,523,600
Available-for-sale Securities, Gross Realized Losses		41,679	49,965	$ 71,893
Securities for Reverse Repurchase Agreements		$ 413,819	$ 476,531
Mortgage Loan Percent Of Invested Assets		8.50%	8.40%
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due		$ 19,156	$ 0
Policy Loans Percent Of Invested Assets		2.60%	3.20%
Funds Withheld At Interest Percent Of Invested Assets		11.80%	13.10%
Percentage Of Total Gross Unrealized Losses		100.00%	100.00%
Funds withheld at interest		$ 6,083,388	$ 5,875,919
Funds Held Under Reinsurance Agreements Asset Specific Client		$ 4,100,000
Other Invested Assets Percent Of Invested Assets		3.10%	3.60%
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total		100.00%	100.00%
Collateral Obtained Percent Of Fair Value Reverse Repurchase Agreements		100.00%
Non-Income Producing Securities [Member]
Investment Narrative [Line Items]
Available-for-sale Securities, Amortized Cost Basis		$ 38,800	$ 35,400
Available-for-sale Securities		39,300	47,300
Canadian Province Of Quebec Securities [Member]
Investment Narrative [Line Items]
Available-for-sale Securities, Amortized Cost Basis		939,837	832,764
Available-for-sale Securities		1,282,944	1,126,433
Reported Value Measurement [Member]
Investment Narrative [Line Items]
Securities Borrowed		358,875	263,820
Funds withheld at interest	[1]	$ 5,955,092	$ 5,893,381
CALIFORNIA
Investment Narrative [Line Items]
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total		19.50%
TEXAS
Investment Narrative [Line Items]
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total		8.40%
GEORGIA
Investment Narrative [Line Items]
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total		7.60%
CANADA
Investment Narrative [Line Items]
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total		2.30%

[1] Carrying values presented herein may differ from those in the Company’s consolidated balance sheets because certain items within the respective financial statement captions may be measured at fair value on a recurring basis.