Stock-Based Compensation, Dividend Restrictions, and Calculations of Earnings Per Share of Common Stock - Weighted Average Grant Date Fair Value Per Share (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value per share of award granted during the year	$ 26.80
Performance-Based Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value per share of award granted during the year	$ 27.01
Unrestricted Stock Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value per share of award granted during the year			$ 18.85
Time-Based Restricted Stock Units Award [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value per share of award granted during the year	$ 19.77	$ 19.70	$ 19.69
Performance Based Restricted Stock Units [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted average grant-date fair value per share of award granted during the year	$ 18.53	$ 17.03	$ 21.13