Investment Securities - Pledged Securities and Concentration (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Pledged securities
Total pledged securities	$ 2,700,000	$ 3,000,000
Securities pledged to secure public deposits	2,486,067	2,800,690
Securities pledged to secure public deposits and repurchase transactions		2,800,000
Securities pledged to secure other financial transactions	229,700	229,200
Non-government issuer
Concentration of risk
Securities of issuers in excess of 10% of stockholders' equity	$ 0	$ 0