(Textual) (Details) (USD $)	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Share-Based Compensation Plans (Textual) [Abstract]
Share-based compensation expense	$ 554,000	$ 629,000
Restricted Stock Units (RSUs) [Member]
Share-Based Compensation Plans (Textual) [Abstract]
Corporation granted options to purchase restricted stock performance units	67,896
Unrecognized compensation expense	3,500,000
Unrecognized compensation expense, period of recognition	2 years 10 months 28 days
Restricted Stock Units (RSUs) [Member] | Maximum [Member]
Share-Based Compensation Plans (Textual) [Abstract]
Restricted stock performance units eligible to vest as a multiple of number of units granted	1.5
Restricted Stock Units (RSUs) [Member] | Minimum [Member]
Share-Based Compensation Plans (Textual) [Abstract]
Restricted stock performance units eligible to vest as a multiple of number of units granted	0.25
Stock Option [Member]
Share-Based Compensation Plans (Textual) [Abstract]
Corporation granted options to purchase common stock	190,011
Common stock available for future grants under share-based compensation plans	420,975
Weighted-average remaining contractual terms for outstanding stock options	6 years 2 months 13 days
Weighted-average remaining contractual terms for exercisable stock options	4 years 3 months 19 days
Outstanding in-the-money stock options, intrinsic value	7,100,000
Exercisable in-the-money stock options, intrinsic value	4,200,000
Closing price of common stock	$ 32.45
Unrecognized compensation expense	$ 3,600,000
Unrecognized compensation expense, period of recognition	4 years 0 months 0 days