(Textual) (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Share-Based Compensation Plans (Textual) [Abstract]
Share-based compensation expense			$ 1,381,000	$ 786,000
Restricted Stock Units (RSUs) [Member]
Share-Based Compensation Plans (Textual) [Abstract]
Share-based compensation expense	700,000	500,000	1,300,000	800,000
Corporation granted options to purchase restricted stock performance units			71,429
Unrecognized compensation expense	3,400,000		3,400,000
Unrecognized compensation expense, period of recognition			2 years 3 months 18 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 10 months 24 days
Weighted-average remaining contractual terms for exercisable stock options			4 years 2 months 12 days
Outstanding in-the-money stock options, intrinsic value	1,900,000		1,900,000
Exercisable in-the-money stock options, intrinsic value	1,200,000		1,200,000
Closing price of common stock	$ 25.99		$ 25.99
Unrecognized compensation expense	2,700,000		2,700,000
Unrecognized compensation expense, period of recognition			2 years 2 months 12 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	$ 1,800,000	$ 1,500,000