Past Due Loans, Allowance For Credit Losses, Impaired Loans, and TDRS - Credit Quality Indicators by Class (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Accruing		$ 1,778	$ 1,796
Special Mention and Substandard Loans
Financing Receivable, Recorded Investment [Line Items]
Accruing		651	657
Total corporate loans
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		8,670,836	8,514,418
Total corporate loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		8,341,391	8,197,560
Total corporate loans | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	181,990	154,601
Total corporate loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	82,613	105,691
Total corporate loans | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	64,842	56,566
Commercial and industrial | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		3,659,066	3,529,914
Accruing		260	264
Commercial and industrial | Commercial and industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		3,505,129	3,388,133
Commercial and industrial | Commercial and industrial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	95,259	70,863
Commercial and industrial | Commercial and industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	14,704	30,338
Commercial and industrial | Commercial and industrial | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	43,974	40,580
Commercial and industrial | Agricultural
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		435,734	430,886
Accruing		0	0
Commercial and industrial | Agricultural | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		417,644	413,946
Commercial and industrial | Agricultural | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	7,756	10,989
Commercial and industrial | Agricultural | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	6,248	5,732
Commercial and industrial | Agricultural | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	4,086	219
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		4,576,036	4,553,618
Accruing		759	766
Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		4,418,618	4,395,481
Commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	78,975	72,749
Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	61,661	69,621
Commercial real estate | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	16,782	15,767
Commercial real estate | Office, retail, and industrial
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		1,931,202	1,979,820
Accruing		0	0
Commercial real estate | Office, retail, and industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		1,856,832	1,903,737
Commercial real estate | Office, retail, and industrial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	26,642	25,546
Commercial real estate | Office, retail, and industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	35,386	38,977
Commercial real estate | Office, retail, and industrial | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	12,342	11,560
Commercial real estate | Multi-family
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		695,830	675,463
Accruing		570	574
Commercial real estate | Multi-family | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		682,926	665,496
Commercial real estate | Multi-family | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	10,961	7,395
Commercial real estate | Multi-family | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	1,799	2,195
Commercial real estate | Multi-family | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	144	377
Commercial real estate | Construction
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		585,766	539,820
Accruing		0	0
Commercial real estate | Construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		568,148	521,911
Commercial real estate | Construction | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	9,941	10,184
Commercial real estate | Construction | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	7,469	7,516
Commercial real estate | Construction | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	208	209
Commercial real estate | Other commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		1,363,238	1,358,515
Accruing		189	192
Commercial real estate | Other commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		1,310,712	1,304,337
Commercial real estate | Other commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	31,431	29,624
Commercial real estate | Other commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	17,007	20,933
Commercial real estate | Other commercial real estate | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	$ 4,088	$ 3,621

[1] Loans categorized as special mention exhibit potential weaknesses that require the close attention of management since these potential weaknesses may result in the deterioration of repayment prospects in the future.

[2] Total special mention and substandard loans includes accruing TDRs of $651,000 as of March 31, 2018 and $657,000 as of December 31, 2017.

[3] Loans categorized as substandard exhibit well-defined weaknesses that may jeopardize the liquidation of the debt. These loans continue to accrue interest because they are well-secured and collection of principal and interest is expected within a reasonable time.

[4] Loans categorized as non-accrual exhibit well-defined weaknesses that may jeopardize the liquidation of the debt or result in a loss if the deficiencies are not corrected.