Summary of significant accounting policies.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Present value of lessee's discounted obligation for lease payments from operating lease.

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- Definition

Description of a specific restructuring plan and related activities including the facts and circumstances leading to the restructuring or decision to exit an activity, and the expected completion date. This concept and related items would be utilized for each plan if multiple exit plans have been implemented in the period.

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Basis of Presentation and Significant Accounting Policies - Additional Information (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
	Apr. 16, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017	Apr. 01, 2019	Apr. 01, 2018
Summary Of Significant Accounting Policies [Line Items]
Net income		$ 295	$ 214	$ 194	$ 92	$ 52	$ 3,076	$ 117	$ 165	$ 795	$ 3,410	$ 267
Accounting Standards Update 2014-09 [Member] \| Adjustments Related to Adoption [Member]
Summary Of Significant Accounting Policies [Line Items]
Net income										$ (5)
Accounting Standards Update 2016-01 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect of adoption of the guidance, reduction in retained earnings														$ 12
Subsequent Event [Member]
Summary Of Significant Accounting Policies [Line Items]
Restructuring and related activities, description	On April 16, 2019, we announced that we will restructure our field operations over the next two years to better serve our dealer partners by streamlining our field office structure into three regional locations and investing in new technology. Costs associated with this restructure are not expected to be significant.
Subsequent Event [Member] \| Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use assets													$ 120
Operating lease liabilities													$ 120
Geographic Concentration Risk [Member] \| Sales Revenue, Net [Member] \| Finance Operations [Member] \| California [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration Percentage										23.00%
Geographic Concentration Risk [Member] \| Sales Revenue, Net [Member] \| Finance Operations [Member] \| Texas [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration Percentage										11.00%
Geographic Concentration Risk [Member] \| Sales Revenue, Net [Member] \| Finance Operations [Member] \| New York [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration Percentage										8.00%
Geographic Concentration Risk [Member] \| Sales Revenue, Net [Member] \| Finance Operations [Member] \| New Jersey [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration Percentage										5.00%
Geographic Concentration Risk [Member] \| Sales Revenue, Net [Member] \| Insurance Operations [Member] \| California [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration Percentage										26.00%
Geographic Concentration Risk [Member] \| Sales Revenue, Net [Member] \| Insurance Operations [Member] \| New York [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration Percentage										6.00%
Geographic Concentration Risk [Member] \| Sales Revenue, Net [Member] \| Insurance Operations [Member] \| New Jersey [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration Percentage										5.00%
Geographic Concentration Risk [Member] \| Sales Revenue, Net [Member] \| Insurance Operations [Member] \| Maryland [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration Percentage										5.00%
Geographic Concentration Risk [Member] \| Sales Revenue, Net [Member] \| Insurance Operations [Member] \| Virginia [Member]
Summary Of Significant Accounting Policies [Line Items]
Concentration Percentage										5.00%