Summary of Significant Accounting Policies - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014
Shares
Options outstanding at end of period (in shares)	24,000	100,762
Exercisable at end of year (in shares)	24,000	164,150	24,000	152,150
Exercise Price
Options outstanding at end of year (in dollars per share)	$ 38.39	$ 37.75
Exercisable at end of year	$ 38.39	$ 38.21	$ 38.39	$ 38.27
Additional information
Remaining Contractual Life - Outstanding Options (in years)	7 months 6 days	10 months 24 days
Remaining Contractual Life - Exercisable Options - (in years)	7 months 6 days	1 year 8 months 12 days	9 months 18 days	1 year
Intrinsic value of unexercised in-the-money options - Outstanding Options (in dollars)	$ 12
Intrinsic value of unexercised in-the-money options - Exercisable Options (in dollars)	$ 12	$ 1,224	$ 22