Derivative Financial Instruments - Summary of Outstanding Interest Rate Derivatives Designated as Cash Flow Hedges of Interest Rate Risk (Detail) - Cash Flow Hedges [Member] - Designated as Hedging Instrument [Member] £ in Thousands, $ in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) DerivativeInstrument	Dec. 31, 2017 USD ($) DerivativeInstrument	Sep. 30, 2018 CAD ($) DerivativeInstrument	Dec. 31, 2017 GBP (£) DerivativeInstrument	Dec. 31, 2017 CAD ($) DerivativeInstrument
Interest Rate Caps [Member] | GBP [Member] | GBP LIBOR [Member]
Derivative [Line Items]
Number of Instruments | DerivativeInstrument		3		3	3
Notional Amount | £				£ 23,370
Strike		2.00%		2.00%	2.00%
Wtd. Avg. Maturity (Years)		3 months 19 days
Interest Rate Caps [Member] | USD [Member] | USD LIBOR [Member]
Derivative [Line Items]
Number of Instruments	9	9	9	9	9
Notional Amount | $	$ 204,248	$ 204,248
Strike	2.40%	2.40%	2.40%	2.40%	2.40%
Wtd. Avg. Maturity (Years)	8 months 12 days	1 year 6 months
Interest Rate Caps [Member] | CAD [Member] | CDOR [Member]
Derivative [Line Items]
Number of Instruments | DerivativeInstrument	3	3	3	3	3
Notional Amount | $			$ 22,765		$ 23,370
Strike	2.00%	2.00%	2.00%	2.00%	2.00%
Wtd. Avg. Maturity (Years)	4 months 24 days	3 months 19 days
Interest Rate Swaps [Member] | CAD [Member] | CDOR [Member]
Derivative [Line Items]
Number of Instruments | DerivativeInstrument	4	4	4	4	4
Notional Amount | $			$ 107,822		$ 108,094
Strike	1.00%	1.00%	1.00%	1.00%	1.00%
Wtd. Avg. Maturity (Years)	8 months 12 days	1 year 4 months 24 days