Interim Financial Data - Impact on Consolidated Statement of Income for the Adoption of Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Summary Of Significant Accounting Policies [Line Items]
Insurance earned premiums and contract revenues	$ 226	$ 221	$ 450	$ 437
Operating and administrative expenses	348	337	672	650
Provision for income taxes	87	70	109	166
Net income	194	117	286	282
Comprehensive income	196	$ 125	284	$ 282
Effect of Adoption [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Insurance earned premiums and contract revenues	1		5
Operating and administrative expenses	1		1
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	227		455
Operating and administrative expenses	$ 349		673
Provision for income taxes			110
Net income			289
Comprehensive income			$ 287