Summary of Significant Accounting Policies - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Shares
Options outstanding at beginning of period (in shares)	24,000	100,762	100,762
Exercised (in Shares)	(6,400)
Expired (in shares)	(12,000)	(12,000)
Options outstanding at end of period (in shares)	5,600	88,762	24,000	100,762
Exercisable at end of year (in shares)	5,600	88,762	24,000	100,762
Exercise Price
Options outstanding at beginning of year (in dollars per share)	$ 38.39	$ 37.75	$ 37.75
Exercised (in dollars per share)	37.17
Expired (in dollars per share)	39.60	35.20
Options outstanding at end of year (in dollars per share)	37.17	38.10	38.39	$ 37.75
Exercisable at end of year	$ 37.17	$ 38.10	$ 38.39	$ 37.75
Additional information
Remaining Contractual Life - Outstanding Options (in years)	7 months 6 days	6 months	9 months 18 days	10 months 24 days
Remaining Contractual Life - Exercisable Options - (in years)	7 months 6 days	6 months	9 months 18 days	10 months 24 days
Intrinsic value of unexercised in-the-money options - Outstanding Options (in dollars)	$ 33		$ 22
Intrinsic value of unexercised in-the-money options - Exercisable Options (in dollars)	$ 33		$ 22