Investment Securities (Narrative) (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($) item security	Jun. 30, 2016 USD ($) security	Jun. 30, 2017 USD ($) item	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) item
Investment Securities [Line Items]
Marketable securities	$ 748,774,000		$ 748,774,000		$ 826,686,000
Income tax expense related to net realized gains		$ 761,000	24,000	$ 761,000
Municipal Security [Member]
Investment Securities [Line Items]
Held-to-maturity securities, Number of sales | security	1
Held-to-maturity securities, carrying value	$ 354,000		$ 354,000
Held-to-maturity securities, realized gain	$ 1,000
Mortgage-Backed Securities [Member]
Investment Securities [Line Items]
Held-to-maturity securities, Number of sales | security		1
Percent of amount collected		85.00%
Held-to-maturity securities, carrying value		$ 106,000		$ 106,000
Held-to-maturity securities, realized gain		$ 17,000
Excludes U.S. Government And Its Agencies [Member]
Investment Securities [Line Items]
Number of holdings greater than 10 percent of stockholders equity | item	0		0		0
Securities Pledged As Collateral [Member]
Investment Securities [Line Items]
Marketable securities	$ 453,295,000		$ 453,295,000		$ 415,419,000