Loans Receivable (Schedule of Consumer Loans Forecasted Collection Percentage) (Details)		Sep. 30, 2018		Jun. 30, 2018	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Loans Originating In 2009 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Collection Percentage		79.50%	[1]	79.50%	79.50%	[1]
Initial Forecasted Collection Percentage	[1]													71.90%
Variance In Forecasted Collection Percentage From Prior Quarter End		0.00%
Variance In Forecasted Collection Percentage From Prior Year End		0.00%
Variance In Initial Forecasted Collection Percentage		7.60%
Loans Originating In 2010 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Collection Percentage		77.70%	[1]	77.70%	77.60%	[1]
Initial Forecasted Collection Percentage	[1]												73.60%
Variance In Forecasted Collection Percentage From Prior Quarter End		0.00%
Variance In Forecasted Collection Percentage From Prior Year End		0.10%
Variance In Initial Forecasted Collection Percentage		4.10%
Loans Originating In 2011 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Collection Percentage		74.70%	[1]	74.80%	74.70%	[1]
Initial Forecasted Collection Percentage	[1]											72.50%
Variance In Forecasted Collection Percentage From Prior Quarter End		(0.10%)
Variance In Forecasted Collection Percentage From Prior Year End		0.00%
Variance In Initial Forecasted Collection Percentage		2.20%
Loans Originating In 2012 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Collection Percentage		73.80%	[1]	73.80%	73.80%	[1]
Initial Forecasted Collection Percentage	[1]										71.40%
Variance In Forecasted Collection Percentage From Prior Quarter End		0.00%
Variance In Forecasted Collection Percentage From Prior Year End		0.00%
Variance In Initial Forecasted Collection Percentage		2.40%
Loans Originating In 2013 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Collection Percentage		73.50%	[1]	73.50%	73.50%	[1]
Initial Forecasted Collection Percentage	[1]									72.00%
Variance In Forecasted Collection Percentage From Prior Quarter End		0.00%
Variance In Forecasted Collection Percentage From Prior Year End		0.00%
Variance In Initial Forecasted Collection Percentage		1.50%
Loans Originating In 2014 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Collection Percentage		71.70%	[1]	71.70%	71.70%	[1]
Initial Forecasted Collection Percentage	[1]								71.80%
Variance In Forecasted Collection Percentage From Prior Quarter End		0.00%
Variance In Forecasted Collection Percentage From Prior Year End		0.00%
Variance In Initial Forecasted Collection Percentage		(0.10%)
Loans Originating In 2015 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Collection Percentage		65.40%	[1]	65.40%	65.50%	[1]
Initial Forecasted Collection Percentage	[1]							67.70%
Variance In Forecasted Collection Percentage From Prior Quarter End		0.00%
Variance In Forecasted Collection Percentage From Prior Year End		(0.10%)
Variance In Initial Forecasted Collection Percentage		(2.30%)
LoansOriginatingin2016 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Collection Percentage		64.30%	[1]	64.40%	64.80%	[1]
Initial Forecasted Collection Percentage	[1]						65.40%
Variance In Forecasted Collection Percentage From Prior Quarter End		(0.10%)
Variance In Forecasted Collection Percentage From Prior Year End		(0.50%)
Variance In Initial Forecasted Collection Percentage		(1.10%)
LoansOriginatingin2017 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Collection Percentage		65.60%	[1]	65.60%	65.60%	[1]
Initial Forecasted Collection Percentage	[1]				64.00%
Variance In Forecasted Collection Percentage From Prior Quarter End		0.00%
Variance In Forecasted Collection Percentage From Prior Year End		0.00%
Variance In Initial Forecasted Collection Percentage		1.60%
LoansOriginatingin2018 [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Forecasted Collection Percentage		64.80%	[1]	64.50%
Initial Forecasted Collection Percentage	[1]	63.60%
Variance In Forecasted Collection Percentage From Prior Quarter End		0.30%
Variance In Initial Forecasted Collection Percentage		1.20%

[1] Represents the total forecasted collections we expect to collect on the Consumer Loans as a percentage of the repayments that we were contractually owed on the Consumer Loans at the time of assignment. Contractual repayments include both principal and interest. Forecasted collection rates are negatively impacted by canceled Consumer Loans as the contractual amount owed is not removed from the denominator for purposes of computing forecasted collection rates in the table.