CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Parenthetical) - $ / shares	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Statement of Stockholders' Equity [Abstract]
Cumulative dividends declared per common share	$ 0.85	$ 1.25	$ 1.20	$ 1.50	$ 2.00