Derivatives (Interest Rate Derivative Contracts) (Details) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Derivative [Line Items]
Derivative assets	$ 81,456,000	$ 66,146,000
Derivative liabilities	114,542,000	74,935,000
Derivatives not designated as hedging instruments
Derivative [Line Items]
Notional amount	12,295,137,000	10,153,108,000
Derivative assets	79,140,000	66,146,000
Derivative liabilities	105,895,000	$ 68,136,000
Derivatives not designated as hedging instruments | Customer Counterparties
Derivative [Line Items]
Derivative assets	3,628
Derivative liabilities	271
Derivatives not designated as hedging instruments | Customer Counterparties | Written Option
Derivative [Line Items]
Notional amount	843,965,000		$ 691,548,000
Derivative assets	0		0
Derivative liabilities	696,000		223,000
Derivatives not designated as hedging instruments | Customer Counterparties | Collars and Corridors
Derivative [Line Items]
Notional amount	335,169,000		247,542,000
Derivative assets	147,000		204,000
Derivative liabilities	1,004,000		267,000
Derivatives not designated as hedging instruments | Customer Counterparties | Swaps
Derivative [Line Items]
Notional amount	4,425,902,000		3,724,295,000
Derivative assets	15,902,000		32,241,000
Derivative liabilities	76,297,000		24,879,000
Derivatives not designated as hedging instruments | Customer Counterparties | Interest rate swaps and options
Derivative [Line Items]
Notional amount	5,605,036,000		4,663,385,000
Derivative assets	16,049,000		32,445,000
Derivative liabilities	77,997,000		25,369,000
Derivatives not designated as hedging instruments | Financial Counterparties
Derivative [Line Items]
Derivative assets	0
Derivative liabilities	2,888
Derivatives not designated as hedging instruments | Financial Counterparties | Purchased Options
Derivative [Line Items]
Notional amount	843,965,000		691,548,000
Derivative assets	711,000		233,000
Derivative liabilities	0		0
Derivatives not designated as hedging instruments | Financial Counterparties | Collars and Corridors
Derivative [Line Items]
Notional amount	335,169,000		247,542,000
Derivative assets	1,015,000		271,000
Derivative liabilities	147,000		211,000
Derivatives not designated as hedging instruments | Financial Counterparties | Swaps
Derivative [Line Items]
Notional amount	4,433,298,000		3,731,385,000
Derivative assets	46,450,000		25,684,000
Derivative liabilities	13,234,000		32,378,000
Derivatives not designated as hedging instruments | Financial Counterparties | Interest rate swaps and options
Derivative [Line Items]
Notional amount	5,612,432,000		4,670,475,000
Derivative assets	48,176,000		26,188,000
Derivative liabilities	$ 13,381,000		$ 32,589,000