Internal Risk Rating Categories for Commercial Portfolio (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	$ 4,539,256	$ 4,355,968
Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	888,528	902,053
Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	3,116,561	3,010,798
Construction Loans [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	401,797	320,391
Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	132,370	122,726
Pass [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	4,345,232	4,097,798
Pass [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	806,331	783,825
Pass [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	3,007,672	2,876,570
Pass [Member] | Construction Loans [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	400,964	317,099
Pass [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	130,265	120,304
Potential weakness [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	87,822	134,039
Potential weakness [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	16,563	46,176
Potential weakness [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	69,788	84,641
Potential weakness [Member] | Construction Loans [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	0	1,363
Potential weakness [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	1,471	1,859
Definite weakness [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	99,516	121,640
Definite weakness [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	59,415	71,991
Definite weakness [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	38,637	47,164
Definite weakness [Member] | Construction Loans [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	833	1,929
Definite weakness [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	631	556
Parital loss probable [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	6,686	2,491
Parital loss probable [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	6,219	61
Parital loss probable [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	464	2,423
Parital loss probable [Member] | Construction Loans [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	0	0
Parital loss probable [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	3	7
Definite loss [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	0	0
Definite loss [Member] | Commercial and Industrial [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	0	0
Definite loss [Member] | Commercial Real Estate [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	0	0
Definite loss [Member] | Construction Loans [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	0	0
Definite loss [Member] | Small Business [Member]
Internal risk-rating categories for the Company's commercial portfolio
Financing Receivable, Net	$ 0	$ 0