Treasury Stock and Share Based Awards - Additional Information (Detail) (USD $)	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Stockholders Equity Note [Line Items]
Intrinsic value of options exercised	$ 168,000
Average remaining contractual life of options outstanding	1 year
Tax benefit recognized on stock option exercised	71,000
Total fair value of vested shares	12,345
Tax benefit recognized on vested shares	3,808
Restricted Stock Units (RSUs)
Stockholders Equity Note [Line Items]
Market value of common stock on the date of grant	$ 29.44
Unrecognized Compensation expense	5,398,000
Weighted average period over which remaining compensation expense will be recognized (in years)	2 years
Number of hypothetical shares of common stock	115,107
Percentage of IRR, which is the threshold	8.50%
Percentage of IRR, which is the target	10.50%
Percentage of IRR, which is the maximum	12.50%
Percentage of restricted stock units vest	100.00%
Expected forfeiture rate	5.00%
Compensation expense	1,665,000	1,471,000
Restricted stock unit awards vested	291,560
Restricted stock unit conversion into share of common stock, conversion ratio	1
Dividends paid on vested restricted stock units	927,000
Total fair value of shares vested during the period	8,593,000
Recognized tax benefits related to shares vested during the period	3,609,000
Number of hypothetical shares of common stock	818,218
Restricted Stock
Stockholders Equity Note [Line Items]
Shares of Restricted Stock Granted	195,459
Market value of common stock on the date of grant	$ 29.39
Minimum forfeiture rate for restricted stock awards	0.00%
Maximum forfeiture rate for restricted stock awards	5.00%
Compensation expense	5,123,000	5,397,000
Unrecognized Compensation expense	$ 18,497,000
Weighted average period over which remaining compensation expense will be recognized (in years)	2 years
Restricted stock unit awards vested	423,215
Restricted Stock | Director
Stockholders Equity Note [Line Items]
Award vesting period for restricted stock granted	1 year
Restricted Stock | Remaining principal due on December 18, 2019
Stockholders Equity Note [Line Items]
Award vesting period for restricted stock granted	3 years
Restricted Stock | Prior to December 15, 2017
Stockholders Equity Note [Line Items]
Award vesting period for restricted stock granted	4 years
Restricted Stock | Minimum
Stockholders Equity Note [Line Items]
Market value of common stock on the date of grant	$ 27.92
Restricted Stock | Maximum
Stockholders Equity Note [Line Items]
Market value of common stock on the date of grant	$ 29.44