Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability - Cumulative Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Medical Professional Liability - Specialty $ 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	$ 4,971
IBNR	995
2008
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	438	$ 442	$ 455	$ 467	$ 468	$ 468	$ 480	$ 496	$ 451	$ 426
IBNR	$ 4
Cumulative Number of Claims | claims	14,102
2009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 422	432	463	471	480	506	494	469	$ 462
IBNR	$ 3
Cumulative Number of Claims | claims	15,594
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 398	403	446	470	486	478	478	$ 483
IBNR	$ 9
Cumulative Number of Claims | claims	15,239
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 491	491	501	533	507	492	$ 486
IBNR	$ 16
Cumulative Number of Claims | claims	17,481
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 563	559	567	575	529	$ 526
IBNR	$ 39
Cumulative Number of Claims | claims	18,503
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 573	567	560	540	$ 534
IBNR	$ 44
Cumulative Number of Claims | claims	19,777
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 564	585	548	$ 511
IBNR	$ 78
Cumulative Number of Claims | claims	19,764
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 543	539	$ 480
IBNR	$ 164
Cumulative Number of Claims | claims	17,690
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 527	$ 469
IBNR	$ 268
Cumulative Number of Claims | claims	14,743
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 452
IBNR	$ 370
Cumulative Number of Claims | claims	11,137