Past Due Loans, Allowance For Credit Losses, Impaired Loans, and TDRS - Credit Quality Indicators by Class (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Accruing		$ 1,760	$ 1,796
Special Mention and Substandard Loans
Financing Receivable, Recorded Investment [Line Items]
Accruing		645	657
Total corporate loans
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		8,786,248	8,514,418
Total corporate loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		8,410,654	8,197,560
Total corporate loans | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	216,469	154,601
Total corporate loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	115,407	105,691
Total corporate loans | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	43,718	56,566
Commercial and industrial | Commercial and industrial
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		3,844,067	3,529,914
Accruing		255	264
Commercial and industrial | Commercial and industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		3,648,626	3,388,133
Commercial and industrial | Commercial and industrial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	122,881	70,863
Commercial and industrial | Commercial and industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	49,888	30,338
Commercial and industrial | Commercial and industrial | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	22,672	40,580
Commercial and industrial | Agricultural
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		433,175	430,886
Accruing		0	0
Commercial and industrial | Agricultural | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		415,043	413,946
Commercial and industrial | Agricultural | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	8,474	10,989
Commercial and industrial | Agricultural | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	6,666	5,732
Commercial and industrial | Agricultural | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	2,992	219
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		4,509,006	4,553,618
Accruing		753	766
Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		4,346,985	4,395,481
Commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	85,114	72,749
Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	58,853	69,621
Commercial real estate | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	18,054	15,767
Commercial real estate | Office, retail, and industrial
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		1,834,918	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		1,770,908	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]	23,602	25,546
Commercial real estate | Office, retail, and industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	31,401	38,977
Commercial real estate | Office, retail, and industrial | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	9,007	11,560
Commercial real estate | Multi-family
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		703,091	675,463
Accruing		566	574
Commercial real estate | Multi-family | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		687,065	665,496
Commercial real estate | Multi-family | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	10,460	7,395
Commercial real estate | Multi-family | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	2,015	2,195
Commercial real estate | Multi-family | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	3,551	377
Commercial real estate | Construction
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		633,601	539,820
Accruing		0	0
Commercial real estate | Construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		607,142	521,911
Commercial real estate | Construction | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[1],[2]	18,915	10,184
Commercial real estate | Construction | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	7,336	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		1,337,396	1,358,515
Accruing		187	192
Commercial real estate | Other commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class		1,281,870	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]	32,137	29,624
Commercial real estate | Other commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[2],[3]	18,101	20,933
Commercial real estate | Other commercial real estate | Non-accrual
Financing Receivable, Recorded Investment [Line Items]
Credit Quality Indicators by Class	[4]	$ 5,288	$ 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 $645,000 as of June 30, 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.