Recent Accounting Pronouncements - Additional Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Jan. 02, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Lease liability	$ 107,272,000	$ 0			$ 0
Lease, right-of-use asset	102,976,000	0			0
Cumulative-effect adjustment, increase in retained earnings					6,322,000	$ (4,179,000)
Derivative notional amount	567,000,000.0				$ 1,482,000,000
Estimated increase in allowance for credit losses	(700,000)	$ 0
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Lease liability				$ 122,900,000
Lease, right-of-use asset				$ 118,700,000
Cumulative-effect adjustment, increase in retained earnings			$ 6,300,000
Accounting Standards Update 2018-16 [Member] | LIBOR [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Derivative notional amount	4,800,000,000
Accounting Standards Update 2016-13 [Member] | Minimum [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Estimated increase in allowance for credit losses	35,000,000
Expected increase in off balance sheet exposures	3,000,000
Accounting Standards Update 2016-13 [Member] | Maximum [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Estimated increase in allowance for credit losses	45,000,000
Expected increase in off balance sheet exposures	$ 8,000,000