Interim Financial Data - Impact on Consolidated Statement of Income for the Adoption of Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Summary Of Significant Accounting Policies [Line Items]
Insurance earned premiums and contract revenues	$ 224	$ 216
Provision for income taxes	22	96
Net income	92	165
Comprehensive income	88	$ 157
Effect of Adoption [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Insurance earned premiums and contract revenues	4
Provision for income taxes	1
Net income	3
Comprehensive income	3
Balances without Adoption [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Insurance earned premiums and contract revenues	228
Provision for income taxes	23
Net income	95
Comprehensive income	$ 91