Fair Value - Changes in Fair Value for Items Measured at Fair Value Pursuant to Election of the Fair Value (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
Schedule of Trading Securities and Other Trading Assets [Line Items]
Interest Income	$ 118,556	$ 108,478	$ 104,333	$ 105,801	$ 109,917	$ 107,803	$ 99,340	$ 85,643	$ 437,168	$ 402,703	$ 366,116
Interest (Expense)	$ (16,578)	$ (15,047)	$ (13,876)	$ (12,667)	$ (11,932)	$ (11,910)	$ (10,903)	$ (9,686)	(58,168)	(44,431)	$ (33,073)
Residential Loans Held for Sale [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Other Gains and (Losses)									409	(103)
Interest Income									4
Interest (Expense)									0	0
Total Changes in Fair Values Included in Current Period Earnings									$ 413	$ (103)