Stockholders' Equity - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator:
Net income available to common shareholders	$ 29,471	$ 20,098	$ 109,401	$ 76,829
Denominator:
Basic weighted average shares outstanding	58,741,973	58,808,098	58,785,546	58,771,444
Effect of dilutive securities:
Diluted weighted average shares outstanding	59,084,117	59,156,432	59,111,303	59,126,994
Basic earnings per share	$ 0.50	$ 0.34	$ 1.86	$ 1.31
Diluted earnings per share	$ 0.50	$ 0.34	$ 1.85	$ 1.30
Stock Appreciation Rights and Options [Member]
Effect of dilutive securities:
Dilutive securities	177,412	194,630	181,680	197,810
Restricted Stock [Member]
Effect of dilutive securities:
Dilutive securities	164,732	153,704	144,077	157,740