Investment Securities - (Narrative) (Details) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($) security	Dec. 31, 2015 USD ($) security
Investment securities total	$ 1,400,000	$ 1,600,000
Amortized cost	1,036,935	1,167,819
Available-for-sale securities	1,046,047	1,157,246
Held-to-maturity securities	$ 381,172	$ 427,503
Mortgage-backed securities as percentage of available-for-sale investment portfolio	100.00%	100.00%
Number of securities | security	42	66
Fair value of available-for-sale securities in an unrealized loss position	$ 476,781	$ 752,663
Pledged as collateral for line of credit at the Federal Home Loan Bank	57,600	0
Fair value of available-for-sale investment securities pledged as collateral	$ 309,300	$ 335,800
Estimated weighted average life of the available-for-sale mortgage-backed securities portfolio	3 years 1 month 6 days	3 years 7 months 6 days
Other securities having no contractual maturity date	$ 400
Number of held-to-maturity securities in unrealized loss positions | security	4	12
Fair value of held-to-maturity securities in an unrealized loss position	$ 36,935	$ 109,856
Held-to-maturity investment securities pledged as collateral	$ 233,700	$ 156,500
Estimated weighted average expected life of the held-to-maturity mortgage-backed securities portfolio	3 years 1 month 6 days	3 years 8 months 12 days
Municipal securities maturing within one to five years [Member]
Amortized cost	$ 4,200
Available-for-sale securities	4,200
Municipal securities due within five to ten years [Member]
Amortized cost	1,000
Available-for-sale securities	$ 1,000