Gain on Sales of Investment Properties (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Sales of investment properties
Sales proceeds	$ 6,918,000	$ 126,785,000	$ 50,563,000
Gain on sale of real estate
Sales of investment properties
Number of properties sold	46	75	44
Sales proceeds	107,200,000	134,200,000	50,600,000
Gain on sales of investment properties	42,100,000	64,700,000	9,900,000
Gain on sale of real estate | Crest Net Lease, Inc. (Crest)
Sales of investment properties
Number of properties sold	1	1
Sales proceeds	$ 820,000	$ 597,000