Condensed Consolidated Balance Sheets (Unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Pledged securities included in debt securities	$ 112,343	$ 108,427
Preferred stock, par value	$ 0.00001	$ 0.00001
Preferred stock, shares authorized	500,000	500,000
Preferred stock, outstanding	0	0
Common stock, par value	$ 0.00001	$ 0.00001
Common stock, shares authorized	300,000,000	300,000,000
Common stock, shares outstanding	111,545,000	110,925,000