Investment Securities - Income Earned on Securities (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Non-taxable:
Total	$ 14,296	$ 9,990	$ 26,918	$ 19,441
Held-to-maturity
Taxable:
Taxable	546	636	1,113	1,297
Non-taxable:
Non-taxable	1,897	2,217	3,833	4,500
Available-for-sale
Taxable:
Taxable	10,218	6,238	19,250	12,054
Non-taxable:
Non-taxable	$ 1,635	$ 899	$ 2,722	$ 1,590