Derivative Instruments and Hedging Activities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Swap	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($) Swap
Derivative Instruments, Gain (Loss) [Line Items]
Reclassified as an increase to interest expense, expected during the next 12 months	$ 5,802,000
Ineffectiveness on cash flow hedges	0
Unrealized gain or loss on derivatives, tax (expense) benefit	(5,735,000)	$ 2,713,000
Termination value of derivatives in a net liability position	8,815,000
Collateral posted relating to credit risk-related contingent features	$ 0
Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Number of interest rate swaps held | Swap	5
Notional amount of interest rate swap	$ 1,000,000,000
Combined Interest Rate Cash Flow Hedges, All Swaps [Member] | Former Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Number of interest rate swaps held | Swap			4
Notional amount of interest rate swap			$ 1,250,000,000
Not Designated as Hedge Accounting Relationships [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives outstanding	$ 0	$ 0