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

12 Months Ended

Dec. 31, 2017

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, 2014	463,893		$	28.96
Granted	181,797		34.77
Forfeited	(14,988	)	34.10
Vested	(187,709	)	25.82
Outstanding as of December 31, 2015	442,993		32.84
Granted	193,734		35.89
Forfeited	(3,044	)	28.62
Vested	(174,891	)	31.47
Outstanding as of December 31, 2016	458,792		34.68
Granted	202,145		41.96
Forfeited	(64,840	)	39.57
Vested	(196,721	)	31.78
Outstanding as of December 31, 2017	399,376		37.98

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:

	2017					2016
Risk-free interest rate				1.5	%				0.9	%
Expected dividend yield				—	%				—	%
Expected term (in years)				3.0					3.0
Volatility	15.6	%	-	22.9	%	14.5	%	-	25.9	%