Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share
Shares not included in computation of diluted weighted average earnings per common													700	0	4,300
Net income (loss) attributable to common shareholders	$ 41,864	$ 4,939	$ 5,657	$ 7,839	$ 2,418	$ 3,985	$ 3,366	$ 4,011	$ 3,037	$ 2,551	$ (486)	$ 1,358	$ 60,299	$ 13,780	$ 6,460
Basic weighted average common shares outstanding	6,090,000	6,055,000	6,036,000	6,114,000	6,149,000	6,307,000	6,685,000	7,149,000	7,739,000	7,839,000	7,841,000	7,848,000	6,074,000	6,570,000	7,817,000
Potentially dilutive common shares													146,000	144,000	170,000
Diluted weighted average common shares outstanding	6,201,000	6,173,000	6,142,000	6,240,000	6,275,000	6,448,000	6,819,000	7,272,000	7,872,000	7,963,000	7,841,000	8,044,000	6,220,000	6,714,000	7,987,000
Basic earnings per common share: (in dollars per share)	$ 6.87	$ 0.82	$ 0.94	$ 1.28	$ 0.40	$ 0.63	$ 0.50	$ 0.57	$ 0.39	$ 0.33	$ (0.06)	$ 0.17	$ 9.93	$ 2.10	$ 0.83
Diluted earnings (loss) per common share (in dollars per share)	$ 6.75	$ 0.80	$ 0.92	$ 1.25	$ 0.39	$ 0.62	$ 0.49	$ 0.55	$ 0.38	$ 0.32	$ (0.06)	$ 0.17	$ 9.69	$ 2.05	$ 0.81