Fair Value of Assets and Liabilities (Financial Instruments where Carrying Amounts and Fair Values May Differ) (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage loans on real estate		$ 4,400,533	$ 3,775,522
Policy loans		1,357,624	1,427,602
Funds withheld at interest		6,083,388	5,875,919
Cash and cash equivalents		1,303,524	1,200,718	$ 1,525,275	$ 1,645,669
Short-term investments		93,304	76,710
Other invested assets		1,605,484	1,591,940
Accrued investment income		392,721	347,173
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Interest-sensitive contract liabilities		16,227,642	14,029,354
Collateral finance and securitization notes		783,938	840,700
Carrying Value [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage loans on real estate		4,400,533	3,775,522
Policy loans		1,357,624	1,427,602
Funds withheld at interest	[1]	5,955,092	5,893,381
Cash and cash equivalents		946,736	862,117
Short-term investments		42,558	32,469
Other invested assets		690,198	477,132
Accrued investment income		392,721	347,173
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Interest-sensitive contract liabilities	[1]	12,683,872	10,225,099
Long And Short Term Debt		2,788,365	3,088,635
Collateral finance and securitization notes		783,938	840,700
Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage loans on real estate		4,477,654	3,786,987
Policy loans		1,357,624	1,427,602
Funds withheld at interest	[1]	6,275,623	6,193,166
Cash and cash equivalents		946,736	862,117
Short-term investments		42,558	32,469
Other invested assets		718,282	510,640
Accrued investment income		392,721	347,173
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Interest-sensitive contract liabilities	[1]	12,917,243	10,234,544
Long And Short Term Debt		2,959,912	3,186,173
Collateral finance and securitization notes		722,145	745,805
Estimate of Fair Value Measurement [Member] | Net Asset Value [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage loans on real estate		0	0
Policy loans		0	0
Funds withheld at interest	[1]	0	0
Cash and cash equivalents		0	0
Short-term investments		0	0
Other invested assets		335,125	296,773
Accrued investment income		0	0
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Interest-sensitive contract liabilities	[1]	0	0
Long And Short Term Debt		0	0
Collateral finance and securitization notes		0	0
Fair Value, Inputs, Level 1 [Member] | Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage loans on real estate		0	0
Policy loans		0	0
Funds withheld at interest	[1]	0	0
Cash and cash equivalents		946,736	862,117
Short-term investments		42,558	32,469
Other invested assets		28,540	26,294
Accrued investment income		0	0
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Interest-sensitive contract liabilities	[1]	0	0
Long And Short Term Debt		0	0
Collateral finance and securitization notes		0	0
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage loans on real estate		0	0
Policy loans		1,357,624	1,427,602
Funds withheld at interest	[1]	0	0
Cash and cash equivalents		0	0
Short-term investments		0	0
Other invested assets		67,778	55,669
Accrued investment income		392,721	347,173
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Interest-sensitive contract liabilities	[1]	0	0
Long And Short Term Debt		0	0
Collateral finance and securitization notes		0	0
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value Measurement [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Mortgage loans on real estate		4,477,654	3,786,987
Policy loans		0	0
Funds withheld at interest	[1]	6,275,623	6,193,166
Cash and cash equivalents		0	0
Short-term investments		0	0
Other invested assets		286,839	131,904
Accrued investment income		0	0
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]
Interest-sensitive contract liabilities	[1]	12,917,243	10,234,544
Long And Short Term Debt		2,959,912	3,186,173
Collateral finance and securitization notes		$ 722,145	$ 745,805

[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.