Other Assets - Loan to a Distribution Partner (Details)	1 Months Ended	3 Months Ended								4 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($) renewal_option	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Apr. 30, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Payments for notes issued											$ 8,000,000	$ 3,073,000	$ 406,000
Total revenue		$ 88,793,000	$ 89,962,000	$ 85,988,000	$ 74,194,000	$ 69,794,000	$ 67,846,000	$ 64,423,000	$ 59,043,000		338,937,000	261,106,000	$ 208,888,000
Distribution Partner Two | Notes Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable, maximum available	$ 4,000,000						$ 4,000,000
Interest rate (percent)	6.00%						6.00%
Number of renewal options | renewal_option	2
Renewal term (years)	1 year
Proceeds from notes receivable			$ 4,200,000
Payments for notes issued										$ 4,000,000
Note receivable		4,000,000				$ 3,000,000				4,000,000	4,000,000	$ 3,000,000
Distribution Partner Three | Notes Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable, maximum available														$ 3,000,000
Interest rate (percent)														8.50%
Distribution Partners Two and Three | Notes Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total revenue											1,200,000
LIBOR | Distribution Partner Two | Notes Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Basis spread on variable rate (percent)	4.00%						4.00%
Other Assets | Distribution Partner Three | Notes Receivable
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes receivable, maximum available		$ 3,000,000								$ 3,000,000	$ 3,000,000