Derivative Financial Instruments - Summary of collars designated as cash flow hedges (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 1,332,000,000	$ 1,482,000,000
Variable Interest Rate Collar [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount	$ 300,000,000	$ 200,000,000
Weighted average cap rates	3.21%	3.44%
Weighted average floor rates	2.21%	2.38%
Weighted average rates	2.49%	2.35%
Weighted average maturity	2 years 7 months 6 days	2 years 9 months 18 days
Unrealized gains (losses)	$ 2,004,000	$ 1,309,000