Investment Securities - Pledged Securities and Concentration (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Pledged securities
Total pledged securities	$ 3,000,000	$ 2,700,000
Securities pledged to secure public deposits	2,800,690	2,521,761
Securities pledged to secure public deposits and repurchase transactions	2,800,000	2,500,000
Securities pledged to secure other financial transactions	229,200	209,100
Non-government issuer
Concentration of risk
Securities of issuers in excess of 10% of stockholders' equity	$ 0	$ 0