Accounting Policies - Components of Other Comprehensive Income Loss and Related Tax Impact (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, before Tax	$ 23,022	$ 18,804
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	4,835	6,581
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	18,187	12,223
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	23,240	12,299
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(2,241)	(4,513)
Other Comprehensive Income (Loss), before Tax	26,581	11,980
Other Comprehensive Income (Loss), Tax	5,582	4,194
Other comprehensive income, net of tax	20,999	7,786
Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, before Tax	6,396	117
Other Comprehensive Income (Loss) before Reclassifications, Tax	1,343	41
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	5,053	76
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(211)	(332)
Other Comprehensive Income (Loss), before Tax	6,663	629
Other Comprehensive Income (Loss), Tax	1,399	221
Other comprehensive income, net of tax	5,264	408
Available-for-sale Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	18,187	12,223
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,417	2,578
Other Comprehensive Income (Loss), before Tax	21,228	14,838
Other Comprehensive Income (Loss), Tax	4,458	5,193
Other comprehensive income, net of tax	16,770	9,645
Held-to-maturity Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,035	2,267
Other Comprehensive Income (Loss), before Tax	(1,310)	(3,487)
Other Comprehensive Income (Loss), Tax	(275)	(1,220)
Other comprehensive income, net of tax	(1,035)	(2,267)
Gain (Loss) on Derivative Instruments [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Available-for-sale Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	0	(3,959)
Reclassification from AOCI, Current Period, Tax	0	(1,386)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	2,573
Other Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Available-for-sale Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(7)	(7)
Reclassification from AOCI, Current Period, Tax	(2)	(2)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	5	5
Interest Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Available-for-sale Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(1,787)	0
Reclassification from AOCI, Current Period, Tax	(375)	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,412	0
Interest Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Held-to-maturity Securities [Member] | Accumulated Net Investment Gain (Loss) Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	(1,310)	(3,487)
Reclassification from AOCI, Current Period, Tax	(275)	(1,220)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,035	2,267
Interest Expense [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Including Portion Attributable to Noncontrolling Interest [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income on Derivatives [Line Items]
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	267	512
Reclassification from AOCI, Current Period, Tax	56	180
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	$ (211)	$ (332)