Equity Financing - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	1 Months Ended				12 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Oct. 31, 2015	Apr. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity [Abstract]
Common stock issued	5.8	2.1	3.3	2.7
Common stock average price per share	$ 59.75	$ 17.84	$ 39.00	$ 38.00
Net proceeds from issue of common stock	$ 327,400	$ 37,300	$ 119,900	$ 94,500	$ 364,802	$ 119,916	$ 94,503
Underwriting discounts, commission and offering expenses	$ 17,600	$ 200	$ 6,800	$ 6,200
Underwriting discount and commission per share		$ 1.60