Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Available-for-sale securities, amortized cost	$ 7,862,311	$ 11,131,008
Held-to-maturity securities	$ 15,188,236	$ 12,548,280
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares, issued	52,586,498	52,835,188
Common stock, shares outstanding	52,586,498	52,835,188