Claim and Claim Adjustment Expense Reserves (International - Hardy - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	$ 4,366	$ 4,167	$ 4,089
Total net liability for unpaid claim and claim adjustment expenses	18,070	18,249	18,576	$ 18,927
International
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	1,432	1,201
Hardy | International
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	916
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	393
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2008	5
Liability for unallocated claim adjustment expenses for accident years presented	8
Total net liability for unpaid claim and claim adjustment expenses	406
Hardy | International | 2008
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	11	11	10	10	$ 14	$ 3
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						15
Hardy | International | 2009
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	(5)	(4)	(4)	(4)	(2)	2
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						12
Hardy | International | 2010
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	49	48	46	43	36	19
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						39
Hardy | International | 2011
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	134	133	129	124	84	31
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						126
Hardy | International | 2012
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	110	108	110	101	81	15
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented						$ 105
Hardy | International | 2013
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	133	129	123	103	$ 39
Hardy | International | 2014
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	153	143	124	$ 56
Hardy | International | 2015
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	131	99	$ 30
Hardy | International | 2016
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	147	$ 64
Hardy | International | 2017
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 53