Derivative Instruments and Hedging Activities - Additional Information (Detail) - USD ($)	1 Months Ended	9 Months Ended
	Aug. 31, 2016	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Net carrying value		$ 1,450,818,000	$ 1,810,728,000
Derivative instruments not designated as hedging instruments, liability				$ 25,000,000
Aggregate fair value of net liability position		69,300,000
Collateral posted with counterparties		92,100,000
Interest Rate Swaps
Derivative [Line Items]
Accumulated net after-tax losses related to effective cash flow hedges		18,600,000	$ 17,600,000
Estimated amount of interest rate derivatives to be reclassified to interest expense		10,400,000
Subordinated debt | Subordinated Notes Due September 2023
Derivative [Line Items]
Face amount of debt		$ 125,000,000
Stated interest rate (as a percent)		5.125%
Effective interest rate (as a percent)		3.32%
Net carrying value		$ 137,100,000
Interest Rate Swaps
Derivative [Line Items]
Notional amount of terminated derivative	$ 125,000,000