Derivatives (Derivatives Associated With Cash Flow Hedges) (Details) - Cash Flow Hedge - Hedging Instruments And Hedged Items - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Mar. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Variability in cash flows related to debt instruments (primarily loans)			$ 650,000	$ 250,000
Interest Rate Contract
Derivative Instruments, Gain (Loss) [Line Items]
Variability in cash flows related to debt instruments (primarily loans)	$ 900,000	$ 900,000
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Notional	900,000	900,000
Cash Flow Hedge Derivative Instrument Assets at Fair Value	690	$ 942
Liabilities	$ 23