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Loans Receivable (Schedule of Consumer Loans Forecasted Collection Percentage) (Details) | Mar. 31, 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
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Loans Originating In 2009 [Member] |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Forecasted Collection Percentage | [1] | 79.50% | 79.50% |  |  |  |  |  |  |  |
Initial Forecasted Collection Percentage | [1] |  |  |  |  |  |  |  |  |  | 71.90%
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 | [1] | 77.60% | 77.60% |  |  |  |  |  |  |  |
Initial Forecasted Collection Percentage | [1] |  |  |  |  |  |  |  |  | 73.60% |
Variance In Forecasted Collection Percentage From Prior Year End |  | 0.00% |  |  |  |  |  |  |  |  |
Variance In Initial Forecasted Collection Percentage |  | 4.00% |  |  |  |  |  |  |  |  |
Loans Originating In 2011 [Member] |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Forecasted Collection Percentage | [1] | 74.70% | 74.70% |  |  |  |  |  |  |  |
Initial Forecasted Collection Percentage | [1] |  |  |  |  |  |  |  | 72.50% |  |
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 | [1] | 73.80% | 73.80% |  |  |  |  |  |  |  |
Initial Forecasted Collection Percentage | [1] |  |  |  |  |  |  | 71.40% |  |  |
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 | [1] | 73.40% | 73.50% |  |  |  |  |  |  |  |
Initial Forecasted Collection Percentage | [1] |  |  |  |  |  | 72.00% |  |  |  |
Variance In Forecasted Collection Percentage From Prior Year End |  | (0.10%) |  |  |  |  |  |  |  |  |
Variance In Initial Forecasted Collection Percentage |  | 1.40% |  |  |  |  |  |  |  |  |
Loans Originating In 2014 [Member] |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Forecasted Collection Percentage | [1] | 71.70% | 71.70% |  |  |  |  |  |  |  |
Initial Forecasted Collection Percentage | [1] |  |  |  |  | 71.80% |  |  |  |  |
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 | [1] | 65.30% | 65.50% |  |  |  |  |  |  |  |
Initial Forecasted Collection Percentage | [1] |  |  |  | 67.70% |  |  |  |  |  |
Variance In Forecasted Collection Percentage From Prior Year End |  | (0.20%) |  |  |  |  |  |  |  |  |
Variance In Initial Forecasted Collection Percentage |  | (2.40%) |  |  |  |  |  |  |  |  |
LoansOriginatingin2016 [Member] |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Forecasted Collection Percentage | [1] | 64.40% | 64.80% |  |  |  |  |  |  |  |
Initial Forecasted Collection Percentage | [1] |  |  | 65.40% |  |  |  |  |  |  |
Variance In Forecasted Collection Percentage From Prior Year End |  | (0.40%) |  |  |  |  |  |  |  |  |
Variance In Initial Forecasted Collection Percentage |  | (1.00%) |  |  |  |  |  |  |  |  |
LoansOriginatingin2017 [Member] |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Forecasted Collection Percentage | [1] | 65.50% | 65.60% |  |  |  |  |  |  |  |
Initial Forecasted Collection Percentage | [1] |  | 64.00% |  |  |  |  |  |  |  |
Variance In Forecasted Collection Percentage From Prior Year End |  | (0.10%) |  |  |  |  |  |  |  |  |
Variance In Initial Forecasted Collection Percentage |  | 1.50% |  |  |  |  |  |  |  |  |
LoansOriginatingin2018 [Member] |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Forecasted Collection Percentage | [1] | 63.60% |  |  |  |  |  |  |  |  |
Initial Forecasted Collection Percentage | [1] | 63.50% |  |  |  |  |  |  |  |  |
Variance In Initial Forecasted Collection Percentage |  | 0.10% |  |  |  |  |  |  |  |  |
[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.

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