Investments Investments (Narrative) (Details) $ in Thousands		9 Months Ended	12 Months Ended
		Sep. 30, 2016	Dec. 31, 2016 USD ($) position	Dec. 31, 2015 USD ($) position	Dec. 31, 2014 USD ($)
Investment Narrative [Line Items]
Net Amount Assets Liabilities From Repurchase Reverse Repurchase Agreements			$ 5,500	$ 8,800
Amount Liabilities From Repurchase Agreements			28,800
Available-for-sale Securities, Amortized Cost Basis			18,924,711	17,575,507
Available-for-sale Securities			$ 19,619,084	$ 17,708,156
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | position			1,535	2,080
Proceeds From Sale Of Available For Sale Securities Sold At Loss			$ 1,181,600	$ 1,523,600	$ 1,016,500
Available-for-sale Securities, Gross Realized Losses			49,965	71,893	$ 31,295
Securities for Reverse Repurchase Agreements			$ 476,531	$ 443,435
Mortgage Loan Percent Of Invested Assets			8.40%	7.50%
Policy Loans Percent Of Invested Assets			3.20%	3.50%
Funds Withheld At Interest Percent Of Invested Assets			13.10%	14.00%
Percentage Of Total Gross Unrealized Losses			100.00%	100.00%
Funds withheld at interest			$ 5,875,919	$ 5,880,203
Funds Held Under Reinsurance Agreements Asset Specific Client			$ 4,000,000
Other Invested Assets Percent Of Invested Assets			3.60%	3.10%
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			$ 35,400	$ 116,000
Available-for-sale Securities			47,300	123,000
Canadian Province Of Quebec Securities [Member]
Investment Narrative [Line Items]
Available-for-sale Securities, Amortized Cost Basis			832,764	864,444
Available-for-sale Securities			1,126,433	1,199,080
Reported Value Measurement [Member]
Investment Narrative [Line Items]
Securities Borrowed			263,820	259,540
Funds withheld at interest	[1]		5,893,381	5,956,380
Mortgage Receivable [Member]
Investment Narrative [Line Items]
Financing Receivable, Recorded Investment, Equal to Greater than 90 Days Past Due			$ 0	$ 0
CALIFORNIA
Investment Narrative [Line Items]
Mortgage Loans On Real Estate Commercial And Consumer Net Percentage Of Total			22.10%	22.30%

[1] Carrying values presented herein differ from those presented in the consolidated balance sheets because certain items within the respective financial statement caption are embedded derivatives and are measured at fair value on a recurring basis.