Investment Securities (Narrative) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($) securities	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) securities	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Securities, Available-for-sale [Line Items]
Securities owned not readily marketable	$ 26,600								$ 26,600
Upward adjustments of equity securities without readily determinable fair values									325	$ 0	$ 0
Downward adjustments of equity securities without readily determinable fair values									0	0	0
Impairment of equity securities without readily determinable fair values									1,117	0	0
Income tax (benefit) expense	28,005	$ 30,866	$ 32,011	$ 26,085	$ 27,004	$ 38,622	$ 37,049	$ 29,640	116,967	132,315	124,979
Investment securities pledged as collateral	$ 1,700,000				$ 1,700,000				$ 1,700,000	1,700,000
Number of securities by a single non-government sponsored issuer exceeding 10% of shareholders' equity | securities	0								0
Available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Income tax (benefit) expense									$ (737)	$ 18	$ 2,900