Employee Compensation Plans - Stock Appreciation Rights (Details)		12 Months Ended
	Dec. 31, 2017 $ / shares shares	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement By Cash Payment Award Stock Appreciation Rights Outstanding Number | shares	2,008,950	2,008,950
Share Based Compensation Arrangement By Cash Payment Award Stock Appreciation Rights unvested amount | shares	1,697,170	1,697,170
Total Number of OARs outstanding | shares	311,780	311,780
Total weighted average strike price	$ 19.35	$ 19.35
Total weighted average remaining contractual life		2 years 2 months
Total weighted average fair value per share	$ 8.93	$ 8.93
January 13, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement By Cash Payment Award Stock Appreciation Rights Outstanding Number | shares	391,730	391,730
Strike price	$ 23.48	$ 23.48
Remaining contractual life	1 year
Fair value per share	$ 4.7546	$ 4.7546
January Nine Two Thousand Fifteen [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement By Cash Payment Award Stock Appreciation Rights Outstanding Number | shares	444,660	444,660
Strike price	$ 21.94	$ 21.94
Remaining contractual life	2 years
Fair value per share	$ 7.0884	$ 7.0884
January Six Two Thousand Seventeen [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement By Cash Payment Award Stock Appreciation Rights Outstanding Number | shares	421,660	421,660
Strike price	$ 18.90	$ 18.90
Remaining contractual life	4 years
Fair value per share	$ 10.3525	$ 10.3525
January Fourteen Two Thousand Thirteen [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement By Cash Payment Award Stock Appreciation Rights Outstanding Number | shares	311,780	311,780
Strike price	$ 15.94	$ 15.94
Remaining contractual life	13 days
Fair value per share	$ 10.8627	$ 10.8627
January Fourteen Two Thousand Thirteen
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share Based Compensation Arrangement By Cash Payment Award Stock Appreciation Rights Outstanding Number | shares	439,120	439,120
Strike price	$ 15.89	$ 15.89
Remaining contractual life	3 years
Fair value per share	$ 11.7673	$ 11.7673