(Textual) (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Share-Based Compensation Plans (Textual) [Abstract]
Share-based compensation expense			$ 2,131,000	$ 1,305,000
Restricted Stock Units (RSUs) [Member]
Share-Based Compensation Plans (Textual) [Abstract]
Share-based compensation expense	800,000	500,000	2,100,000	1,300,000
Corporation granted options to purchase restricted stock performance units			71,429
Unrecognized compensation expense	2,700,000		2,700,000
Unrecognized compensation expense, period of recognition			2 years 2 months 12 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			244,219
Common stock available for future grants under share-based compensation plans	674,140		674,140
Weighted-average remaining contractual terms for outstanding stock options			5 years 8 months 12 days
Weighted-average remaining contractual terms for exercisable stock options			3 years 9 months 18 days
Outstanding in-the-money stock options, intrinsic value	3,300,000		3,300,000
Exercisable in-the-money stock options, intrinsic value	1,800,000		1,800,000
Closing price of common stock	$ 27.92		$ 27.92
Unrecognized compensation expense	2,400,000		2,400,000
Unrecognized compensation expense, period of recognition			2 years 0 months 0 days
Parent Company [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Defined Contribution Plan, Employer Total Contribution Amount	$ 900,000	$ 800,000	$ 2,700,000	$ 2,300,000