Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 10, 2017
Class of Stock [Line Items]
Common stock, shares authorized		240,000,000	240,000,000
Preferred stock, shares authorized		5,000,000
Preferred stock, par value per share (in dollars per share)		$ 0.01
Net proceeds from issuance of common stock		$ 219,504	$ 96,547	$ 0
Common stock, no par, 240,000 shares authorized, 147,740 and 120,630 shares issued, respectively		$ 1,284,274	1,055,934
Underwritten Public Equity Offering [Member] | Common Stock [Member]
Class of Stock [Line Items]
Shares issued					26,450,000
Price per share issued (in dollars per share)					$ 8.65
Net proceeds from issuance of common stock	$ 220,000
ATM Program [Member]
Class of Stock [Line Items]
Net proceeds from issuance of common stock			96,500
Common stock, no par, 240,000 shares authorized, 147,740 and 120,630 shares issued, respectively			100,000
Payments for commissions			$ 2,300
ATM Program [Member] | Common Stock [Member]
Class of Stock [Line Items]
Shares issued			13,018,732