Noncontrolling Interest - Additional Information (Detail) - USD ($) $ in Millions	1 Months Ended	6 Months Ended	12 Months Ended
	Jan. 31, 2013	Jun. 30, 2018	Dec. 31, 2013
Preferred Stock Series B [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Preferred stock issued	$ 7.9
Dividends rate	5.30%
Voting rights		The preferred stock has no voting rights
Preferred Stock Series C [Member]
Consolidation, Less than Wholly Owned Subsidiary, Parent Ownership Interest, Effects of Changes, Net [Line Items]
Preferred stock issued			$ 2.8
Dividends rate			5.30%
Voting rights		The preferred stock has no voting rights