Stock Option Plans - Additional information (Detail) (USD $)	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Mar. 12, 2014	Mar. 13, 2013	Mar. 16, 2012
Stock Option Plans [Line Items]
Maximum number of shares available	376,541	369,975	361,400
Maximum number of shares granted during any fiscal year to any person	200,000
Maximum period for the exercise of stock options, years	7 years
Minimum exercise price of stock options, percent of fair market value of a share of common stock on the date of grant	100.00%
Number of years options expire, maximum	7 years	7 years	7 years
Compensation expense related to stock option grants	$ 9,500,000	$ 7,700,000	$ 7,200,000
Total intrinsic value of options exercised	30,500,000	32,000,000	26,000,000
Total unrecognized compensation cost related to nonvested options	28,800,000
Weighted average period, years	4 years
Minimum [Member]
Stock Option Plans [Line Items]
Period of Service from Grant Date Stock Options Awarded Not Subject to Forfeiture	2 years
Officer and Key Employees [Member]
Stock Option Plans [Line Items]
Shares available for grant				1,923,000	1,665,000	1,355,000
Stock options granted, exercise percentage, on the anniversary date of the grant in 2017				34.00%	34.00%	34.00%
Stock options granted, exercise percentage, on the anniversary date of the grant in 2018				33.00%	33.00%	33.00%
Stock options granted, exercise percentage, on the anniversary date of the grant in 2019				33.00%	33.00%	33.00%
Executive Officer [Member]
Stock Option Plans [Line Items]
Minimum age of employee with not subject to award forfeiture on condition compliance	55 years
Long Term Incentive Plan [Member]
Stock Option Plans [Line Items]
Maximum number of shares available	2,000,000
Shares available for grant	9,300,000
Performance-Based Restricted Stock [Member]
Stock Option Plans [Line Items]
Maximum number of shares granted during any fiscal year to any person	100,000
Incentive Plan Prior to 2007 [Member]
Stock Option Plans [Line Items]
Stock options granted, exercise rate per year	10.00%
Non Qualified Plan Prior to 2007 [Member] | Officer and Key Employees [Member]
Stock Option Plans [Line Items]
Stock options granted, exercise rate per year	10.00%
Accelerated vesting in the event of death, disability or retirement	100.00%
Prior to 2009 Options [Member]
Stock Option Plans [Line Items]
Number of years options expire, maximum	10 years
Cash Settled Performance Shares [Member] | Performance-Based Restricted Stock [Member]
Stock Option Plans [Line Items]
Maximum amount payable for the shares granted during any fiscal year to any person	$ 5,000,000
Stock Settled Performance Shares [Member] | Performance-Based Restricted Stock [Member]
Stock Option Plans [Line Items]
Maximum number of shares granted during any fiscal year to any person	100,000
Black-Scholes Option Pricing Model [Member]
Stock Option Plans [Line Items]
Weighted average fair value per option for all options	$ 9.66	$ 7.51	$ 5.49