Stockholders' Equity (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Gross proceeds from issuance of common stock	$ 212,900,000
Net proceeds from issuance of stock	211,774,000
Aggregate value of units redeemed	$ 0	$ 0
Preferred stock, par value (in usd per share)	$ 1	$ 1
Authorized repurchased amount (up to)			$ 100,000,000
Number of shares repurchased	6,800,000
Total cost of common stock repurchases, including broker commissions	$ 61,500,000
Percentage of restriction on ownership (more than)	3.90%
Common Stock
Class of Stock [Line Items]
Number of shares issued during period	25,000,000
Stock issuance costs	$ 1,100,000
Net proceeds from issuance of stock	$ 211,800,000
Number of units redeemed in exchange for common stock (in shares)	1,203,286
Aggregate value of units redeemed	$ 10,100,000
Preferred Stock
Class of Stock [Line Items]
Number of shares issued in merger		6,900,000