Common Stock (Stock Option Activity and Additional Details) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted in period (in shares)	0	0	0
Shares (in shares)
Outstanding, beginning of period (in shares)	663,288
Exercised (in shares)	(663,288)	(114,112)
Forfeited (in shares)	0
Expired (in shares)	0
Outstanding, end of period (in shares)	0	663,288
Weighted-Average Exercise Price (in dollars per share)
Outstanding, beginning of period (in dollars per share)	$ 8.87
Exercised (in dollars per share)	8.87
Forfeited (in dollars per share)	0.00
Expired (in dollars per share)	0.00
Outstanding, end of period (in dollars per share)	$ 0.00	$ 8.87
Weighted Average Remaining Conctracual Term (in years)
Outstanding	0 days
Aggregate Intrinsic Value
Outstanding	$ 0
Stock Options, Additional Disclosures
Total intrinsic value of stock options exercised	$ 2,300,000	$ 1,000,000	$ 8,400,000
Number of stock options vested (in shares)	0	0	0
Stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award expiration period	10 years
Vesting period in years	4 years