Supplemental Schedule of Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums, Life Insurance in Force	$ 1,462,000	$ 1,576,000	$ 1,686,000
Ceded Premiums, Life Insurance in Force	205,529,000	214,727,000	222,388,000
Assumed Premiums, Life Insurance in Force	3,297,275,000	3,062,525,000	2,995,079,000
Premiums, Net, Life Insurance in Force	$ 3,093,208,000	$ 2,849,374,000	$ 2,774,377,000
Life Insurance in Force Premiums, Percentage Assumed to Net	106.60%	107.50%	108.00%
Direct Premiums Earned	$ 61,571	$ 57,562	$ 43,106
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	862,903	858,278	843,673
Assumed Premiums Earned	10,642,462	10,049,587	9,371,308
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 9,841,130	$ 9,248,871	$ 8,570,741
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	108.10%	108.70%	109.30%
US Traditional Segment Reporting Information [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned	$ 30,500	$ 31,600	$ 29,400
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	610,400	616,000	607,000
Assumed Premiums Earned	5,936,200	5,834,000	5,384,300
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 5,356,300	$ 5,249,600	$ 4,806,700
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	110.80%	111.10%	112.00%
US Non-Traditional Segment Reporting Information [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned	$ 4,300	$ 1,600	$ 2,900
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	0	40,200	38,800
Assumed Premiums Earned	19,400	63,000	58,100
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 23,700	$ 24,400	$ 22,200
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	81.90%	258.20%	261.70%
Canada Traditional Segment Reporting Information [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned	$ 0	$ 0	$ 0
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	38,100	36,500	42,300
Assumed Premiums Earned	940,100	965,100	881,200
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 902,000	$ 928,600	$ 838,900
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	104.20%	103.90%	105.00%
Canada Non-Traditional Segment Reporting Information [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned	$ 0	$ 0	$ 0
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	0	0	0
Assumed Premiums Earned	38,200	38,700	38,000
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 38,200	$ 38,700	$ 38,000
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	100.00%	100.00%	100.00%
EMEA Traditional Segment Reporting Information [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned	$ 26,600	$ 24,100	$ 10,700
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	34,900	30,900	25,500
Assumed Premiums Earned	1,310,000	1,146,900	1,136,300
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 1,301,700	$ 1,140,100	$ 1,121,500
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	100.60%	100.60%	101.30%
EMEA Non-Traditional Segment Reporting Information [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned	$ 200	$ 300	$ 100
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	125,000	84,400	89,100
Assumed Premiums Earned	288,500	264,400	260,800
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 163,700	$ 180,300	$ 171,800
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	176.20%	146.60%	151.80%
Asia Pacific Traditional Segment Reporting Information [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned	$ 0	$ 0	$ 0
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	54,500	50,300	41,000
Assumed Premiums Earned	2,107,500	1,731,800	1,592,600
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 2,053,000	$ 1,681,500	$ 1,551,600
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	102.70%	103.00%	102.60%
Asia Pacific Non-Traditional Segment Reporting Information [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned	$ 0	$ 0	$ 0
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	0	0	0
Assumed Premiums Earned	2,400	5,400	19,500
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 2,400	$ 5,400	$ 19,500
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	100.00%	100.00%	100.00%
Corporate And Other Segment Reporting Information [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct Premiums Earned	$ 0	$ 0	$ 0
Reinsurance Premiums for Insurance Companies, by Product Segment, Amount Ceded to Other Companies	0	0	0
Assumed Premiums Earned	100	300	500
Reinsurance Premiums for Insurance Companies, by Product Segment, Net Amount	$ 100	$ 300	$ 500
Reinsurance Premiums for Insurance Companies, by Product Segment, Percentage of Amount Assumed to Net	100.00%	100.00%	100.00%