Business Acquisitions - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Millions	Jul. 29, 2016	Dec. 31, 2017	Nov. 01, 2017	Dec. 31, 2016	Dec. 31, 2015
Liabilities
Goodwill		$ 1,290		$ 567	$ 498
Minimum
Liabilities
Estimated Useful Life	5 years
Maximum
Liabilities
Estimated Useful Life	8 years
Aetna Group Insurance
Assets
Cash and invested assets			$ 3,360
Premiums receivable			96
Deferred income taxes, net			56
Other intangible assets			629
Property and equipment			68
Other assets			16
Total Assets Acquired			4,225
Liabilities
Unpaid losses and loss adjustment expenses			2,833
Reserve for future policy benefits			346
Other policyholder funds and benefits payable			245
Unearned premiums			3
Other liabilities			69
Total Liabilities Assumed			3,496
Net identifiable assets acquired			729
Goodwill			723
Net Assets Acquired			1,452
Goodwill, expected tax deductible			$ 623
Maxum
Assets
Cash and invested assets	$ 274
Reinsurance recoverables	113
Cash	12
Other intangible assets	7
Intangible assets	11
Other assets	79
Total Assets Acquired	477
Liabilities
Unpaid losses	235
Unearned premiums	77
Other liabilities	34
Total Liabilities Assumed	346
Net identifiable assets acquired	131
Goodwill	38
Net Assets Acquired	169
Indefinite lived intangible assets	$ 4
Estimated Useful Life	10 years
Lattice
Assets
Cash	$ 1
Other intangible assets	1
Intangible assets	11
Total Assets Acquired	12
Liabilities
Total Liabilities Assumed	1
Net identifiable assets acquired	11
Goodwill	31
Net Assets Acquired	42
Indefinite lived intangible assets	$ 10