Accounting Policies - Components of Other Comprehensive Income Loss and Related Tax Impact (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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	$ 36,809	$ 8,433	$ (9,270)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	12,884	2,952	(3,244)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	23,925	5,481	(6,026)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	24,077	55,062	(7,181)
Reclassification from Accumulated Other Comprehensive Income, Current Period, before Tax	1,800	2,000	1,200
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(15,803)	(10,285)	(19,371)
Other Comprehensive Income (Loss), before Tax	12,729	68,889	(40,850)
Other Comprehensive Income (Loss), Tax	4,455	24,112	(14,298)
Other comprehensive income, net of tax	8,274	44,777	(26,552)
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	233	2,443	(1,776)
Other Comprehensive Income (Loss) before Reclassifications, Tax	81	855	(621)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	152	1,588	(1,155)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,178	1,313	803
Other Comprehensive Income (Loss), before Tax	2,046	4,463	(541)
Other Comprehensive Income (Loss), Tax	716	1,562	(189)
Other comprehensive income, net of tax	1,330	2,901	(352)
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	23,925	5,481	(6,026)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(10,917)	(9,833)	(13,725)
Other Comprehensive Income (Loss), before Tax	20,012	(6,694)	(30,387)
Other Comprehensive Income (Loss), Tax	7,004	(2,342)	(10,636)
Other comprehensive income, net of tax	13,008	(4,352)	(19,751)
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, before Tax	0	73,835	0
Other Comprehensive Income (Loss) before Reclassifications, Tax	0	25,842	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	47,993	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(6,064)	(1,765)	(6,449)
Other Comprehensive Income (Loss), before Tax	(9,329)	71,120	(9,922)
Other Comprehensive Income (Loss), Tax	(3,265)	24,892	(3,473)
Other comprehensive income, net of tax	(6,064)	46,228	(6,449)
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	(16,845)	(15,375)	(20,125)
Reclassification from AOCI, Current Period, Tax	(5,897)	(5,381)	(7,044)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(10,948)	(9,994)	(13,081)
Gain (Loss) on Investments [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	(89)	9	(10)
Reclassification from AOCI, Current Period, Tax	(31)	3	(4)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(58)	6	(6)
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	137	239	(982)
Reclassification from AOCI, Current Period, Tax	48	84	(344)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	89	155	(638)
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	(9,329)	(2,715)	(9,922)
Reclassification from AOCI, Current Period, Tax	(3,265)	(950)	(3,473)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	(6,064)	(1,765)	(6,449)
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	1,813	2,020	1,235
Reclassification from AOCI, Current Period, Tax	635	707	432
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	$ 1,178	$ 1,313	$ 803