Derivatives (Narrative) (Details) Boe in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Boe warrant	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) warrant
Derivative [Line Items]
Derivative liabilities	$ 86,673		$ 74,935
Derivative assets	59,831		66,146
Derivative asset, reduction in fair value	38,877		29,205
Derivative liability, reduction in fair value	25,538		48,203
Collateral posted	12,038		28,796
Derivatives designated as hedging instruments
Derivative [Line Items]
Notional amount	133,275		35,811
Derivative liabilities	9,405		6,799
Derivative assets	0		0
Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount	11,133,117		10,153,108
Derivative liabilities	77,268		68,136
Derivative assets	59,831		66,146
Interest rate swaps | Derivatives designated as hedging instruments
Derivative [Line Items]
Notional amount	35,811		35,811
Derivative liabilities	8,251		6,799
Derivative assets	0		0
Interest rate swaps | Derivatives designated as hedging instruments | Fair Value Hedges
Derivative [Line Items]
Notional amount	35,800		35,800
Derivative liabilities	8,300		6,800
Foreign exchange forwards | Derivatives designated as hedging instruments
Derivative [Line Items]
Notional amount	97,464		0
Derivative liabilities	1,154		0
Derivative assets	0		0
Foreign exchange forwards | Derivatives designated as hedging instruments | Net investment hedges
Derivative [Line Items]
Notional amount	97,500
Derivative liabilities	1,200
Total fair value change in Foreign Currency Translation Adjustment with AOCI	(1,200)
Loss recognized in AOCI on net investment hedges (effective portion)		$ 648
(Losses) gains recognized in foreign exchange income (ineffective portion)		$ (2,000)
Interest rate swaps and options
Derivative [Line Items]
Notional amount of derivative assets	5,110,000		4,690,000
Notional amount of derivative liabilities	5,050,000		4,650,000
Interest rate swaps and options | Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount	10,155,401		9,333,860
Derivative liabilities	72,893		57,958
Derivative assets	51,933		58,633
Interest rate swaps and options | Derivatives not designated as hedging instruments | LCH
Derivative [Line Items]
Notional amount of derivative assets	1,400,000
Fair value of interest rate swaps cleared through LCH	0
Derivative asset, reduction in fair value	(25,700)
Derivative liability, reduction in fair value	(3,000)
Foreign exchange spot and forwards | Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount	895,788		770,215
Derivative liabilities	4,066		10,170
Derivative assets	6,087		5,840
Credit risk participation agreements | Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount	81,928		49,033
Derivative liabilities	21		8
Derivative assets	$ 1		$ 1
Weighted average remaining maturity (in years)	6 years 8 months 12 days		6 years
Credit risk participation agreements | Derivatives not designated as hedging instruments | Long
Derivative [Line Items]
Notional amount	$ 68,900		$ 35,200
Derivative liabilities	21		8
Interest rate derivative exposure	756		419
Credit risk participation agreements | Derivatives not designated as hedging instruments | Short
Derivative [Line Items]
Notional amount	13,000		13,800
Derivative assets	1		1
Equity warrants | Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount	0		0
Derivative liabilities	0		0
Derivative assets	1,513		1,672
Warrant, Public Companies | Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets	$ 582		$ 993
Number of warrants held | warrant	4		4
Warrant, Private Companies | Derivatives not designated as hedging instruments
Derivative [Line Items]
Derivative assets	$ 931		$ 679
Number of warrants held | warrant	24		23
Commodity options | Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount	$ 0		$ 0
Derivative liabilities	288		0
Derivative assets	$ 297		0
Commodity options | Derivatives not designated as hedging instruments | Customers
Derivative [Line Items]
Notional amount of commodity contracts in number of barrels of oil | Boe	216
Commodity options | Derivatives not designated as hedging instruments | Chicago Mercantile Exchange (CME)
Derivative [Line Items]
Derivative asset, reduction in fair value	$ (276)
Derivative liability, reduction in fair value	$ (297)
Notional amount of commodity contracts in number of barrels of oil | Boe	216
Credit-risk-related contingent features
Derivative [Line Items]
Aggregate fair value of derivative instruments in net liability position	$ 14,900		6,300
Collateral posted	12,300		$ 6,200
Additional collateral required to be posted if credit rating is downgraded	$ 3,500