Basis and Summary of Significant Accounting Policies - Narrative (Details) £ in Millions, $ in Millions	Mar. 31, 2018 GBP (£) state	Mar. 31, 2018 USD ($) state	Dec. 31, 2017 GBP (£)	Dec. 31, 2017 USD ($)
Progressive Leasing
Significant Accounting Policies [Line Items]
Number of states in which entity operates	46	46
Variable Interest Entity, Not Primary Beneficiary
Significant Accounting Policies [Line Items]
Maximum exposure	£ 14.6	$ 20.5	£ 15.1	$ 20.4