Stockholders' Equity - Reconciliation of beginning and ending common stock outstanding (Details) - shares	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reconciliation of the beginning and ending common stock outstanding
Balance, beginning of period	125,132,000
Balance, end of period	125,198,000
Common Stock
Reconciliation of the beginning and ending common stock outstanding
Balance, beginning of period	125,131,593	116,416,900
Issuance of common stock	11,158	8,395,607
Nonvested share-based awards, net of withheld shares	55,566	319,086
Balance, end of period	125,198,317