Income Taxes Components of Provision for Income Taxes (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Current provision:
Federal									$ 263,231		$ 195,249	$ 191,194
State									67,046		59,319	50,815
Deferred expense (benefit):
Federal									24,654		(3,560)	(11,270)
State									532		(675)	(1,985)
Income tax expense	$ 135,051	$ 97,351	$ 71,656	$ 51,405	$ 54,825	$ 76,877	$ 65,047	$ 53,584	$ 355,463	[1]	$ 250,333	$ 228,754

[1] Included in income tax expense, net income available to common stockholders, earnings per common share-basic and earnings for common share-diluted, for the year ended December 31, 2017, are tax benefits recognized associated with the adoption of Accounting Standards Update ("ASU") 2016-09, Improvements to Employee Share-Based Payment Accounting in the first quarter of 2017. This guidance was adopted on a prospective basis with no change to prior period amounts.