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		Jun. 30, 2018		Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Assets
Mortgage loans on real estate (net of valuation allowances)		$ 4,558,669		$ 4,400,533
Policy loans		1,339,252		1,357,624
Funds withheld at interest		5,981,092		6,083,388
Cash and cash equivalents		1,397,679		1,303,524	$ 1,123,350	$ 1,200,718
Short-term investments		123,028		93,304
Other invested assets		1,605,562		1,505,332
Accrued investment income		400,160		392,721
Liabilities
Interest-sensitive contract liabilities		16,513,668		16,227,642
Long-term debt		2,788,111		2,788,365
Collateral finance and securitization notes		724,998		783,938
Reported Value Measurement [Member]
Assets
Mortgage loans on real estate (net of valuation allowances)	[1]	4,558,669		4,400,533
Policy loans	[1]	1,339,252		1,357,624
Funds withheld at interest	[1]	5,836,373		5,955,092
Cash and cash equivalents	[1]	973,078		946,736
Short-term investments	[1]	40,507		42,558
Other invested assets	[1]	757,264		651,792
Accrued investment income	[1]	400,160		392,721
Liabilities
Interest-sensitive contract liabilities	[1]	13,072,239		12,683,872
Long-term debt	[1]	2,788,111
Debt, Long-term and Short-term, Combined Amount	[1]			2,788,365
Collateral finance and securitization notes	[1]	724,998		783,938
Estimate of Fair Value Measurement [Member]
Assets
Mortgage loans on real estate (net of valuation allowances)		4,461,317		4,477,654
Policy loans		1,339,252		1,357,624
Funds withheld at interest		6,057,217		6,275,623
Cash and cash equivalents		973,078		946,736
Short-term investments		40,507		42,558
Other invested assets		775,322		679,377
Accrued investment income		400,160		392,721
Liabilities
Interest-sensitive contract liabilities		12,972,203	[1]	12,917,243
Long-term Debt, Fair Value		2,868,837
Debt, Long-term and Short-term, Combined Amount				2,959,912
Collateral finance and securitization notes		666,356		722,145
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		0		0
Cash and cash equivalents		973,078		946,736
Short-term investments		40,507		42,558
Other invested assets		5,565		28,540
Accrued investment income		0		0
Liabilities
Interest-sensitive contract liabilities		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		1,339,252		1,357,624
Funds withheld at interest		0		0
Cash and cash equivalents		0		0
Short-term investments		0		0
Other invested assets		71,797		67,778
Accrued investment income		400,160		392,721
Liabilities
Interest-sensitive contract liabilities		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)		4,461,317		4,477,654
Policy loans		0		0
Funds withheld at interest		6,057,217		6,275,623
Cash and cash equivalents		0		0
Short-term investments		0		0
Other invested assets		323,142		247,934
Accrued investment income		0		0
Liabilities
Interest-sensitive contract liabilities		12,972,203		12,917,243
Long-term Debt, Fair Value		2,868,837
Debt, Long-term and Short-term, Combined Amount				2,959,912
Collateral finance and securitization notes		666,356		722,145
Estimate of Fair Value Measurement [Member] | Fair Value Measured at Net Asset Value Per Share [Member]
Assets
Mortgage loans on real estate (net of valuation allowances)		0		0
Policy loans		0		0
Funds withheld at interest		0		0
Cash and cash equivalents		0		0
Short-term investments		0		0
Other invested assets		374,818		335,125
Accrued investment income		0		0
Liabilities
Interest-sensitive contract liabilities		0		0
Long-term Debt, Fair Value		0
Debt, Long-term and Short-term, Combined Amount				0
Collateral finance and securitization notes		$ 0		$ 0

[1] aid.June 30, 2018: Carrying Value (1) Estimated Fair Value Fair Value Measurement Using:Level 1 Level 2 Level 3 NAVAssets: Mortgage loans on real estate $4,558,669 $4,461,317 $— $— $4,461,317 $—Policy loans 1,339,252 1,339,252 — 1,339,252 — —Funds withheld at interest 5,836,373 6,057,217 — — 6,057,217 —Cash and cash equivalents 973,078 973,078 973,078 — — —Short-term investments 40,507 40,507 40,507 — — —Other invested assets 757,264 775,322 5,565 71,797 323,142 374,818Accrued investment income 400,160 400,160 — 400,160 — —Liabilities: Interest-sensitive contract liabilities $13,072,239 $12,972,203 $— $— $12,972,203 $—Long-term debt 2,788,111 2,868,837 — — 2,868,837 —Collateral finance and securitization notes 724,998 666,356 — — 666,356 — December 31, 2017: Carrying Value (1) Estimated Fair Value Fair Value Measurement Using:Level 1 Level 2 Level 3 NAVAssets: Mortgage loans on real estate $4,400,533 $4,477,654 $— $— $4,477,654 $—Policy loans 1,357,624 1,357,624 — 1,357,624 — —Funds withheld at interest 5,955,092 6,275,623 — — 6,275,623 —Cash and cash equivalents 946,736 946,736 946,736 — — —Short-term investments 42,558 42,558 42,558 — — —Other invested assets 651,792 679,377 28,540 67,778 247,934 335,125Accrued investment income 392,721 392,721 — 392,721 — —Liabilities: Interest-sensitive contract liabilities $12,683,872 $12,917,243 $— $— $12,917,243 $—Long-term debt 2,788,365 2,959,912 — — 2,959,912 —Collateral finance and securitization notes 783,938 722,145 — — 722,145 — (1)Carrying values presented herein may differ from those in the Company’s condensed consolidated balance sheets because certain items within the respective financial statement captions may be measured at fair value on a recurring basis.