Interim Financial Data - Impact on Consolidated Statement of Income for the Adoption of Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Summary Of Significant Accounting Policies [Line Items]
Insurance earned premiums and contract revenues	$ 226	$ 220	$ 676	$ 657
Provision for income taxes	69	(2,821)	178	(2,655)
Net income	214	3,076	500	3,358
Comprehensive income	$ 226	$ 3,061	510	$ 3,343
Effect of Adoption [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Insurance earned premiums and contract revenues			6
Provision for income taxes			1
Net income			5
Comprehensive income			5
Balances without Adoption [Member] | Accounting Standards Update 2014-09 [Member]
Summary Of Significant Accounting Policies [Line Items]
Insurance earned premiums and contract revenues			682
Provision for income taxes			179
Net income			505
Comprehensive income			$ 515