Shareholders Equity and Dividend Restrictions (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory Accounting Practices [Line Items]
Statutory net income	$ 2.5	$ 2.0	$ 2.1
Statutory surplus	10.4	$ 8.5	$ 8.0
Minimum statutory surplus required by regulators	3.2
Investments on deposit with regulatory bodies	0.6
Maximum dividend distributions permitted in 2018 without regulatory approval	1.6
Maximum loans to parent company permitted without regulatory approval	1.3
Restricted GAAP net assets of Cigna Corporation's subsidiaries	$ 12.0