Bank Call Loans - Summary of Bank Call Loans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Bank Call Loans [Abstract]
Year-end balance	$ 118,300	$ 145,800
Weighted interest rate (at end of year)	2.25%	1.77%
Maximum balance (at any month-end)	$ 230,400	$ 151,900
Bank Call Loans Average Amount Outstanding	$ 123,918	$ 106,455
Average interest rate (during the year)	2.08%	1.52%