Commitments and Contingencies (Professional Liability Claims Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Malpractice Insurance [Line Items]
Malpractice loss contingency, historical claims period used by actuary to estimate liability	20 years
Malpractice Loss Contingency, Discount Rate	2.20%	1.80%	1.60%
Malpractice Loss Contingency, Accrual, Discounted	$ 711	$ 788
Malpractice Loss Contingency, Accrual, Undiscounted	760	843
Malpractice Loss Contingency, Accrual, Discounted, Current	$ 115	$ 130
Malpractice loss contingency, settled claims as percent of total liability	1.00%
Professional and general liability self-insured claims reported prior to June 1, 2002, per occurrence [Member]
Malpractice Insurance [Line Items]
Malpractice loss contingency, self-insured retention	$ 1
Professional and general liability self-insured reported from June 1, 2002 through June 1, 2003, per occurrence [Member]
Malpractice Insurance [Line Items]
Malpractice loss contingency, self-insured retention	2
Professional and general liability self-insured reported from June 1, 2003 and before June 1, 2005, per occurrence [Member]
Malpractice Insurance [Line Items]
Malpractice loss contingency, self-insured retention	4
Professional and general liability self-insured claims reported on or after June 1, 2005, per occurrence [Member]
Malpractice Insurance [Line Items]
Malpractice loss contingency, self-insured retention	5
Professional and general liability claims reported on or after June 1, 2014 [Member]
Malpractice Insurance [Line Items]
Malpractice loss contingency, self-insured retention	10
Professional and general liability claims reported on or after June 1, 2003 [Member]
Malpractice Insurance [Line Items]
Malpractice Insurance, Maximum Coverage Per Incident	95
Professional and general liability claims reported on or after June 1, 2015 [Member]
Malpractice Insurance [Line Items]
Malpractice Insurance, Maximum Coverage Per Incident	220
Integrated occurrence claims reported on or after June 1, 2014 [Member]
Malpractice Insurance [Line Items]
Malpractice Insurance, Annual Coverage Limit	50
Integrated occurrence claims reported on or after June 1, 2015 [Member]
Malpractice Insurance [Line Items]
Malpractice Insurance, Annual Coverage Limit	75
Professional and general liability claims incurred and reported after January 1, 2008 [Member]
Malpractice Insurance [Line Items]
Malpractice Insurance, Maximum Coverage Per Incident	145
Professional and general liability claims incurred and reported after January 1, 2010 [Member]
Malpractice Insurance [Line Items]
Malpractice Insurance, Maximum Coverage Per Incident	195
Professional and general liability self-insured claims under certain policy terms until Company total aggregate coverage is met if first aggregate layer becomes fully utilized [Member]
Malpractice Insurance [Line Items]
Malpractice loss contingency, self-insured retention	10
Triad professional and general liability self-insured claims during 2007 [Member]
Malpractice Insurance [Line Items]
Malpractice loss contingency, self-insured retention	$ 10
Minimum [Member]
Malpractice Insurance [Line Items]
Malpractice loss contingency, average lag period from occurence to settlement	3 years
Malpractice loss contingency, self-insured retention	$ 10
Minimum [Member] | Triad professional and general liability self-insured claims after December 31, 2006 [Member]
Malpractice Insurance [Line Items]
Malpractice loss contingency, self-insured retention	$ 1
Maximum [Member]
Malpractice Insurance [Line Items]
Malpractice loss contingency, average lag period from occurence to settlement	4 years
Malpractice loss contingency, self-insured retention	$ 15
Maximum [Member] | Triad professional and general liability self-insured claims after December 31, 2006 [Member]
Malpractice Insurance [Line Items]
Malpractice loss contingency, self-insured retention	$ 5