Claim and Claim Adjustment Expense Reserves (International - Excluding Hardy - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Excluding Hardy - International $ in Millions	Dec. 31, 2017 USD ($) claims	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 2,676
IBNR	563
2008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	224	$ 225	$ 229	$ 227	$ 218	$ 226	$ 226	$ 234	$ 246	$ 243
IBNR	$ 7
Cumulative Number of Claims | claims	25,185
2009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 232	233	242	252	256	271	270	268	$ 272
IBNR	$ 7
Cumulative Number of Claims | claims	21,402
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 202	208	215	222	232	239	244	$ 247
IBNR	$ 13
Cumulative Number of Claims | claims	22,943
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 232	235	242	253	275	283	$ 281
IBNR	$ 11
Cumulative Number of Claims | claims	25,448
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 259	267	266	273	289	$ 282
IBNR	$ 28
Cumulative Number of Claims | claims	25,450
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 273	278	298	307	$ 304
IBNR	$ 43
Cumulative Number of Claims | claims	24,276
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 296	308	308	$ 292
IBNR	$ 68
Cumulative Number of Claims | claims	25,343
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 321	323	$ 307
IBNR	$ 88
Cumulative Number of Claims | claims	23,563
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 320	$ 301
IBNR	$ 120
Cumulative Number of Claims | claims	17,166
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 317
IBNR	$ 178
Cumulative Number of Claims | claims	14,772