Equity - Additional Information (Detail)						3 Months Ended
	Mar. 27, 2019 USD ($) shares	Mar. 11, 2019 shares	Mar. 04, 2019 USD ($)	Jun. 21, 2018 USD ($) shares	Mar. 03, 2017 USD ($)	Mar. 31, 2019 USD ($) Tranche shares	Dec. 31, 2018 shares
Stockholders Equity Note Disclosure [Line Items]
Unrecognized compensation expense | $						$ 5,100,000
Aggregate offering price of shares of common stock that the Company may issue and sell | $					$ 100,000,000
Common Stock, shares issued						68,005,907	60,849,206
Underwritten Public Offering [Member]
Stockholders Equity Note Disclosure [Line Items]
Common Stock, shares issued				20,700,000
Shares offered directly by the company				13,700,000
Underwriters option to purchase additional shares				2,700,000
Gross proceeds from shares sold to underwriters | $				$ 252,900,000
Forward sales agreements settlement date						Jun. 21, 2019
Forward Sales Agreement [Member]
Stockholders Equity Note Disclosure [Line Items]
Common Stock, shares issued	6,700,000			7,000,000
Proceeds from issuance of common stock | $	$ 119,200,000
Number of shares remain avaliable under forward sale agreements	300,000
2015 Equity Incentive Plan [Member]
Stockholders Equity Note Disclosure [Line Items]
Compensation expense recognized | $						$ 700,000
2015 Equity Incentive Plan [Member] | Employee [Member]
Stockholders Equity Note Disclosure [Line Items]
Restricted common stock issued		3,080
Restricted common stock grants, vesting description						The shares of restricted common stock will vest upon the second anniversary of the grant date so long as the grantee remains an employee of the Company on such date.
2017 and 2019 ATM Programs [Member]
Stockholders Equity Note Disclosure [Line Items]
Aggregate offering price of shares of common stock that the Company may issue and sell | $			$ 200,000,000
2019 ATM Program [Member]
Stockholders Equity Note Disclosure [Line Items]
Number of shares sold						0
ATM Program [Member]
Stockholders Equity Note Disclosure [Line Items]
Gross sale of common stock available for grant | $						$ 225,800,000
2017 ATM Program [Member]
Stockholders Equity Note Disclosure [Line Items]
Number of shares sold						366,455
Gross sale of common stock available for grant | $						$ 25,800,000
Vesting on January 18, 2021 [Member]
Stockholders Equity Note Disclosure [Line Items]
Restricted stock granted						17,645
Vesting on January 18, 2022 [Member]
Stockholders Equity Note Disclosure [Line Items]
Restricted stock granted						36,396
Long Term Incentive Plan [Member]
Stockholders Equity Note Disclosure [Line Items]
Number of tranches | Tranche						2
Percentage of awards vesting when earned following completion of applicable performance period						50.00%
Percentage of awards earned subject to additional year of vesting						50.00%
Additional vesting year of awards						1 year
Restricted stock granted						143,538
Long Term Incentive Plan [Member] | 2015 Equity Incentive Plan [Member]
Stockholders Equity Note Disclosure [Line Items]
Aggregate performance-based units granted						143,538
Long Term Incentive Plan [Member] | Tranche One [Member]
Stockholders Equity Note Disclosure [Line Items]
Aggregate performance-based units granted						45,238
Long Term Incentive Plan [Member] | Tranche Two [Member]
Stockholders Equity Note Disclosure [Line Items]
Aggregate performance-based units granted						98,300
Restricted Common Stock [Member]
Stockholders Equity Note Disclosure [Line Items]
Restricted stock granted						57,121
Restricted Common Stock [Member] | 2015 Equity Incentive Plan [Member]
Stockholders Equity Note Disclosure [Line Items]
Restricted stock granted						54,041