Compensation Plans - Summary of stock options activities (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common Shares Subject to Option
Beginning balance (shares)	641	661	1,640
Granted (shares)	0	0	0
Exercised (shares)	(20)	0	(2)
Cancelled (shares)	(290)	(20)	(977)
Ending balance (shares)	331	641	661
Exercisable (shares)	264
Weighted- Average Exercise Prices
Weighted-Average Exercise Prices, Beginning balance (USD per share)	$ 24.80	$ 24.97	$ 24.91
Weighted-Average Exercise Prices, Granted (USD per share)	0.00	0.00	0.00
Weighted-Average Exercise Prices, Exercised (USD per share)	22.75	0.00	21.66
Weighted-Average Exercise Prices, Cancelled (USD per share)	26.98	30.49	24.88
Weighted-Average Exercise Prices, Ending balance (USD per share)	23.03	$ 24.80	$ 24.97
Weighted-Average Exercise Prices, Exercisable (USD per share)	$ 23.07
Weighted-Average Remaining Contractual Term	1 year
Weighted-Average Remaining Contractual Term, Exercisable	1 year
Aggregate Intrinsic Value, Exercised	$ 65	$ 0	$ 6
Aggregate Intrinsic Value, Outstanding	1,156
Aggregate Intrinsic Value, Exercisable	$ 911