Subsequent Events - Additional Information (Detail) - USD ($) shares in Thousands, $ in Thousands		1 Months Ended	6 Months Ended
	Jul. 22, 2016	Jul. 31, 2016	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Subsequent Event [Line Items]
Common stock, shares issued			240,341		236,744
Proceeds from sale of common shares / units, net of offering costs			$ 44,306	$ 479,902
2016 Senior Unsecured Notes [Member]
Subsequent Event [Line Items]
Senior unsecured notes, interest rate			6.375%
2016 Senior Unsecured Notes [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Senior unsecured notes face amount	$ 500,000
Senior unsecured notes, maturity date	Aug. 01, 2026
Senior unsecured notes, interest rate	5.25%
Debt instrument, redemption price percentage	105.25%
Senior notes, repurchased price percentage on principal amount plus accrued and unpaid interest	101.00%
2016 Senior Unsecured Notes [Member] | Subsequent Event [Member] | Maximum [Member]
Subsequent Event [Line Items]
Senior unsecured notes, redemption percentage on principal amount	35.00%
2011 Bonds [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Senior unsecured notes face amount	$ 450,000
2026 Senior Unsecured Notes [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Redemption period, end date	Aug. 12, 2016
Debt redemption value	$ 23,000
Senior unsecured notes, payable term	Interest on the notes will be payable on February 1 and August 1 of each year
Senior unsecured notes commencing date of payment	Feb. 01, 2017
Senior notes, earliest redemption date	Aug. 01, 2021
Senior unsecured notes, redemption description	We may redeem some or all of the notes at a premium that will decrease over time. In addition, at any time prior to August 1, 2019, we may redeem up to 35% of the notes at a redemption price equal to 105.25% of the aggregate principal amount thereof, plus accrued and unpaid interest thereon, using proceeds from one or more equity offerings. In the event of a change in control, each holder of the notes may require us to repurchase some or all of the notes at a repurchase price equal to 101% of the aggregate principal amount of the notes plus accrued and unpaid interest to the date of purchase.
Market Equity Offering Program [Member]
Subsequent Event [Line Items]
Common stock, shares issued			3,000
Proceeds from sale of common shares / units, net of offering costs			$ 45,000
Market Equity Offering Program [Member] | Subsequent Event [Member]
Subsequent Event [Line Items]
Common stock, shares issued		2,700
Proceeds from sale of common shares / units, net of offering costs		$ 41,100