Investment Securities - (Narrative) (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Investment securities total	$ 1,200,000	$ 1,100,000
Amortized cost	942,450	870,849
Available-for-sale securities	917,865	855,345
Held-to-maturity securities	$ 283,369	$ 258,730
Mortgage-backed securities as percentage of available-for-sale investment portfolio	100.00%	100.00%
Number of securities | security	205	87
Fair value of available-for-sale securities in an unrealized loss position	$ 792,697	$ 674,091
Fair value of available-for-sale investment securities pledged as collateral	$ 427,800	$ 334,600
Estimated weighted average life of the available-for-sale mortgage-backed securities portfolio	3 years 6 months	3 years 4 months 24 days
Life of the available-for-sale mortgage-backed securities portfolio	3 years 1 month 6 days	3 years 1 month 6 days
Other securities having no contractual maturity date	$ 800
Number of held-to-maturity securities in unrealized loss positions | security	48	36
Fair value of held-to-maturity securities in an unrealized loss position	$ 236,587	$ 219,412
Held-to-maturity investment securities pledged as collateral	$ 140,800	$ 142,000
Estimated weighted average expected life of the held-to-maturity mortgage-backed securities portfolio	3 years 1 month 6 days	3 years 1 month 6 days
Life of the held-to-maturity investment portfolio	2 years 7 months 6 days	2 years 9 months 18 days
Municipal securities maturing within one to five years [Member]
Amortized cost	$ 200
Available-for-sale securities	200
Municipal securities due within five to ten years [Member]
Amortized cost	500
Available-for-sale securities	500
Other Securities [Member]
Amortized cost	469	$ 419
Available-for-sale securities	$ 469	$ 419