v3.8.0.1
Consolidated Statement of Stockholders' Equity (Parenthetical) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Issuance of membership units, number of units 161,000 97,000 793,000
Redemption of shares of common stock, shares 119,000 109,000 43,000
Number of membership units exchanged 37,000 0 0
Common Stock      
Issuance of shares of common stock, shares 3,833,000 3,784,000 4,535,000
Redemption of shares of common stock, shares 119,000 109,000 43,000
Cancellation shares of common stock, shares     14,000
Issuance of shares common stock upon conversion of membership units 37,000