Basis and Summary of Significant Accounting Policies - Investments (Details) - Variable Interest Entity, Not Primary Beneficiary £ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2017 GBP (£) Store	Sep. 30, 2017 USD ($) Store	Dec. 31, 2016 GBP (£)	Dec. 31, 2016 USD ($)
Significant Accounting Policies [Line Items]
Number of retail stores | Store		18	18
Held-to-maturity Securities		£ 17.1	$ 22,900,000	£ 16.6	$ 20,500,000
Investment interest rate	12.00%
Asset impairment charges | $	$ 0
Investments, present value of estimated future cash flows		120.00%	120.00%