Computation of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				12 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income	$ 119,259		$ 115,254		$ 115,254
Less: Dividends paid on and earnings allocated to participating securities	(872)		(796)
Earnings applicable to common stock	$ 118,387		$ 114,458
Weighted average common shares outstanding	441,990,338		440,570,598
Basic earnings per common share	$ 0.27		$ 0.26
Potential dilutive common shares	0	[1]	0	[1]
Total shares for diluted earnings per share computation	441,990,338		440,570,598
Diluted earnings per common share and common share equivalents	$ 0.27		$ 0.26

[1] Options to purchase 32,400 and 57,400 shares, of the Company's common stock that were outstanding as of March 31, 2015 and 2014, respectively, at weighted average exercise prices of $18.15 and $18.08 per share, were excluded from the respective computations of diluted EPS because their inclusion would have had an antidilutive effect.