Investment Securities - (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($) security
Investment securities total	$ 1,200,000	$ 1,600,000
Amortized cost	894,737	1,167,819
Available-for-sale securities	884,232	1,157,246
Held-to-maturity securities	$ 332,505	$ 427,503
Mortgage-backed securities as percentage of available-for-sale investment portfolio	100.00%	100.00%
Number of securities | security	61	66
Fair value of available-for-sale securities in an unrealized loss position	$ 627,239	$ 752,663
Fair value of available-for-sale investment securities pledged as collateral	$ 373,700	$ 335,800
Estimated weighted average life of the available-for-sale mortgage-backed securities portfolio	3 years 4 months 24 days	3 years 7 months 6 days
Life of the available-for-sale mortgage-backed securities portfolio	3 years 2 months 12 days	3 years 4 months 24 days
Number of held-to-maturity securities in unrealized loss positions | security	15	16
Fair value of held-to-maturity securities in an unrealized loss position	$ 86,937	$ 109,856
Held-to-maturity investment securities pledged as collateral	$ 119,200	$ 156,500
Estimated weighted average expected life of the held-to-maturity mortgage-backed securities portfolio	3 years 6 months	3 years 8 months 12 days
Life of the held-to-maturity investment portfolio	3 years 2 months 12 days	3 years 4 months 24 days
Municipal securities maturing within one to five years [Member]
Amortized cost	$ 3,300
Available-for-sale securities	3,300
Municipal securities due within five to ten years [Member]
Amortized cost	600
Available-for-sale securities	600
Other Securities [Member]
Amortized cost	419	$ 419
Available-for-sale securities	$ 419	$ 419