Stock-based Compensation Expense Restricted and Performance Stock Unit activity (Tables)

12 Months Ended

Dec. 31, 2014

Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures [Abstract]

Schedule of Share-based Compensation, Restricted Stock Units Award Activity [Table Text Block]

RSU activity is summarized in the following table:

	Units		Weighted Average Grant Date Fair Value
Outstanding as of December 31, 2011	491,404		$	18.54
Granted	186,495		24.72
Forfeited	(22,947	)	18.95
Vested	(214,390	)	15.67
Outstanding as of December 31, 2012	440,562		22.54
Granted	183,071		29.25
Forfeited	(7,007	)	27.15
Vested	(185,536	)	20.20
Outstanding as of December 31, 2013	431,090		26.31
Granted	203,410		31.49
Forfeited	(12,278	)	29.90
Vested	(158,329	)	24.95
Outstanding as of December 31, 2014	463,893		28.96

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

The fair value of stock-based compensation related to the TSR component of performance-based RSUs was determined using the Monte Carlo model and the following weighted average assumptions:

	2014				2013
Risk-free interest rate			0.6	%			0.3	%
Expected dividend yield			—	%			—	%
Expected term (in years)			3.0				3.0
Volatility	12.4%	-	23.0%		12.1%	-	25.1%