Investment Securities - Pledged Securities and Concentration (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Pledged securities
Total pledged securities	$ 3,700,000	$ 2,700,000
Securities pledged to secure public deposits	3,440,535	2,521,761
Securities pledged to secure public deposits and repurchase transactions		2,500,000
Securities pledged to secure other financial transactions	230,600	209,100
Non-government issuer
Concentration of risk
Securities of issuers in excess of 10% of stockholders' equity	$ 0	$ 0