Investments in Qualified Affordable Housing Projects, Federal Historic Projects and New Market Tax Credits (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Variable Interest Entity [Line Items]
Expense related to qualified affordable housing projects	$ 1,100,000	$ 800,000	$ 2,100,000	$ 1,500,000
Remaining investment in qualified affordable housing projects	53,300,000		53,300,000		$ 51,400,000
Equity method investments	13,900,000		13,900,000		17,300,000
Remaining unfunded equity contributions	44,200,000		44,200,000		48,100,000
Investment tax credit	1,900,000		3,400,000
Impairment on tax credit projects	1,700,000		3,400,000	$ 0	$ 0
Other Noninterest Expense [Member]
Variable Interest Entity [Line Items]
Impairment on tax credit projects	$ 1,400,000		$ 2,600,000