Investment Securities (Narrative) (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) security position	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) security position
Schedule of Held-to-maturity Securities [Line Items]
Available for sale debt securities, at fair value	$ 1,039,304,000		$ 1,037,154,000
Investment securities held to maturity	$ 466,945,000		$ 477,652,000
Total number of all held to maturity and available for sale securities in an unrealized loss position | security	580		306
Amortized cost	$ 1,057,119,000		$ 1,041,868,000
Proceeds from the sale of securities available for sale	$ 0	$ 0
Number of securities in an unrealized loss position | position	177		122
Proceeds from calls of held to maturity securities	$ 20,300,000	12,800,000
Recognized gain on calls of securities, held-to-maturity portfolio	1,000	0
Recognized loss on calls of securities held-to-maturity portfolio	0	$ 0
Investment securities held to maturity, fair value	$ 465,787,000		$ 485,039,000
Number of securities in an unrealized loss position | security	403		184
Private Label Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	$ 32,000
Number of securities in an unrealized loss position | position	1
Unrealized losses	$ 384
Mortgage-Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Available for sale debt securities, at fair value	1,003,603,000		$ 988,367,000
Investment securities held to maturity	311,000		382,000
Amortized cost	1,021,819,000		993,548,000
Held-to-maturity securities, amortized cost	311,000
Investment securities held to maturity, fair value	$ 322,000		$ 396,000