Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	$ 3,239,824	$ 3,338,605
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	38,590,883	33,610,927
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	4,992,074	4,263,026
Supplementary Insurance Information, Premium Revenue	9,841,130	9,248,871	$ 8,570,741
Supplementary Insurance Information, Net Investment Income	2,154,651	1,911,886	1,734,495
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	9,020,957	8,358,066	7,826,346
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	502,866	381,192	229,876
Supplementary Insurance Information, Other Operating Expense	1,849,131	1,738,307	1,617,161
US Traditional Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	1,818,572	1,818,211
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	11,343,921	10,990,560
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	1,814,018	1,659,473
Supplementary Insurance Information, Premium Revenue	5,356,321	5,249,571	4,806,706
Supplementary Insurance Information, Net Investment Income	728,073	699,833	636,779
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	4,842,412	4,717,850	4,444,196
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	191,725	177,255	131,439
Supplementary Insurance Information, Other Operating Expense	692,600	698,762	653,620
US Non-Traditional Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	405,623	582,031
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	15,127,529	13,074,231
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	17,876	20,352
Supplementary Insurance Information, Premium Revenue	23,683	24,349	22,177
Supplementary Insurance Information, Net Investment Income	778,473	631,097	566,180
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	458,368	333,107	310,464
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	185,280	133,501	40,416
Supplementary Insurance Information, Other Operating Expense	105,146	90,458	85,022
Canada Traditional Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	212,345	201,149
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	3,041,790	2,713,510
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	206,655	200,246
Supplementary Insurance Information, Premium Revenue	901,976	928,642	838,894
Supplementary Insurance Information, Net Investment Income	189,018	178,927	182,621
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	757,912	707,428	670,477
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	12,426	10,621	11,299
Supplementary Insurance Information, Other Operating Expense	212,964	265,250	217,061
Canada Non-Traditional Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	0	0
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	27,908	29,531
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	1,040	4,599
Supplementary Insurance Information, Premium Revenue	38,229	38,701	37,969
Supplementary Insurance Information, Net Investment Income	5,115	2,692	1,436
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	29,639	36,275	29,251
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	0	0	0
Supplementary Insurance Information, Other Operating Expense	2,656	2,718	1,881
EMEA Traditional Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	229,150	206,837
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	1,075,786	913,351
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	882,744	740,218
Supplementary Insurance Information, Premium Revenue	1,301,640	1,140,062	1,121,540
Supplementary Insurance Information, Net Investment Income	55,511	50,301	51,370
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	1,096,211	999,005	979,225
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	20,570	33,795	39,164
Supplementary Insurance Information, Other Operating Expense	174,808	132,290	123,943
EMEA Non-Traditional Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	0	0
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	4,741,983	3,457,196
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	38,044	46,761
Supplementary Insurance Information, Premium Revenue	163,720	180,271	171,830
Supplementary Insurance Information, Net Investment Income	123,258	125,282	73,432
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	153,874	178,014	161,917
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	0	0	0
Supplementary Insurance Information, Other Operating Expense	33,683	24,497	16,304
Asia Pacific Traditional Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	552,947	512,123
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	1,611,633	1,288,642
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	2,017,920	1,563,707
Supplementary Insurance Information, Premium Revenue	2,053,029	1,681,505	1,551,586
Supplementary Insurance Information, Net Investment Income	91,675	83,049	80,549
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	1,635,728	1,345,951	1,208,984
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	91,477	24,597	7,373
Supplementary Insurance Information, Other Operating Expense	334,695	286,674	316,346
Asia Pacific Non-Traditional Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	21,187	18,254
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	1,118,012	641,614
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	6,885	19,643
Supplementary Insurance Information, Premium Revenue	2,419	5,428	19,474
Supplementary Insurance Information, Net Investment Income	34,529	23,648	18,678
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	40,467	37,976	20,766
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	1,388	1,423	185
Supplementary Insurance Information, Other Operating Expense	18,790	19,920	16,011
Corporate And Other Segment Reporting Information [Member]
Supplementary Insurance Information, by Segment [Line Items]
Supplementary Insurance Information, Deferred Policy Acquisition Costs	0	0
Supplementary Insurance Information, Liability for Future Policy Benefits, Losses, Claims and Loss Expense Reserves	502,321	502,292
Supplementary Insurance Information, Other Policy Claims and Benefits Payable	6,892	8,027
Supplementary Insurance Information, Premium Revenue	113	342	565
Supplementary Insurance Information, Net Investment Income	148,999	117,057	123,450
Supplementary Insurance Information, Benefits, Claims, Losses and Settlement Expense	6,346	2,460	1,066
Supplementary Insurance Information, Amortization of Deferred Policy Acquisition Costs	0	0	0
Supplementary Insurance Information, Other Operating Expense	$ 273,789	$ 217,738	$ 186,973