Fair Value of Assets and Liabilities Fair Value of Assets and Liabilities (Financial Instruments where Carrying Amounts and Fair Values May Differ) (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Assets
Mortgage loans on real estate (net of valuation allowances)		$ 4,322,329	$ 3,775,522
Policy loans		1,340,146	1,427,602
Funds withheld at interest		6,020,336	5,875,919
Cash and cash equivalents		1,204,590	1,200,718	$ 1,379,693	$ 1,525,275
Short-term investments		80,582	76,710
Other invested assets		1,532,523	1,591,940
Accrued investment income		420,111	347,173
Liabilities
Interest-sensitive contract liabilities		16,370,090	14,029,354
Long-term debt		2,788,480	3,088,635
Collateral finance and securitization notes		796,825	840,700
Reported Value Measurement [Member]
Assets
Mortgage loans on real estate (net of valuation allowances)		4,322,329	3,775,522
Policy loans		1,340,146	1,427,602
Funds withheld at interest	[1]	5,931,416	5,893,381
Cash and cash equivalents	[2]	872,404	862,117
Short-term investments	[2]	24,128	32,469
Other invested assets	[2]	601,905	477,132
Accrued investment income		420,111	347,173
Liabilities
Interest-sensitive contract liabilities	[1]	12,616,104	10,225,099
Long-term debt		2,778,480
Debt, Long-term and Short-term, Combined Amount			3,088,635
Collateral finance and securitization notes		796,825	840,700
Estimate of Fair Value Measurement [Member]
Assets
Mortgage loans on real estate (net of valuation allowances)		4,494,964	3,786,987
Policy loans		1,340,146	1,427,602
Funds withheld at interest	[1]	6,309,369	6,193,166
Cash and cash equivalents	[2]	872,404	862,117
Short-term investments	[2]	24,128	32,469
Other invested assets	[2]	627,650	510,640
Accrued investment income		420,111	347,173
Liabilities
Interest-sensitive contract liabilities	[1]	12,960,161	10,234,544
Long-term Debt, Fair Value		2,997,791
Debt, Long-term and Short-term, Combined Amount			3,186,173
Collateral finance and securitization notes		708,342	745,805
Estimate of Fair Value Measurement [Member] | Net Asset Value [Member]
Assets
Mortgage loans on real estate (net of valuation allowances)		0	0
Policy loans		0	0
Funds withheld at interest	[1]	0	0
Cash and cash equivalents	[2]	0	0
Short-term investments	[2]	0	0
Other invested assets	[2]	300,686	296,773
Accrued investment income		0	0
Liabilities
Interest-sensitive contract liabilities	[1]	0	0
Long-term Debt, Fair Value		0
Debt, Long-term and Short-term, Combined Amount			0
Collateral finance and securitization notes		0	0
Estimate of Fair Value Measurement [Member] | Fair Value Inputs Level 1 Member
Assets
Mortgage loans on real estate (net of valuation allowances)		0	0
Policy loans		0	0
Funds withheld at interest	[1]	0	0
Cash and cash equivalents	[2]	872,404	862,117
Short-term investments	[2]	24,128	32,469
Other invested assets	[2]	28,267	26,294
Accrued investment income		0	0
Liabilities
Interest-sensitive contract liabilities	[1]	0	0
Long-term Debt, Fair Value		0
Debt, Long-term and Short-term, Combined Amount			0
Collateral finance and securitization notes		0	0
Estimate of Fair Value Measurement [Member] | Fair Value Inputs Level 2 Member
Assets
Mortgage loans on real estate (net of valuation allowances)		0	0
Policy loans		1,340,146	1,427,602
Funds withheld at interest	[1]	0	0
Cash and cash equivalents	[2]	0	0
Short-term investments	[2]	0	0
Other invested assets	[2]	68,342	55,669
Accrued investment income		420,111	347,173
Liabilities
Interest-sensitive contract liabilities	[1]	0	0
Long-term Debt, Fair Value		0
Debt, Long-term and Short-term, Combined Amount			0
Collateral finance and securitization notes		0	0
Estimate of Fair Value Measurement [Member] | Fair Value Inputs Level 3 Member
Assets
Mortgage loans on real estate (net of valuation allowances)		4,494,964	3,786,987
Policy loans		0	0
Funds withheld at interest	[1]	6,309,369	6,193,166
Cash and cash equivalents	[2]	0	0
Short-term investments	[2]	0	0
Other invested assets	[2]	230,355	131,904
Accrued investment income		0	0
Liabilities
Interest-sensitive contract liabilities	[1]	12,960,161	10,234,544
Long-term Debt, Fair Value		2,997,791
Debt, Long-term and Short-term, Combined Amount			3,186,173
Collateral finance and securitization notes		$ 708,342	$ 745,805

[1] Carrying values presented herein differ from those presented in the condensed 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.

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