Parent Company Information (Components of Condensed Comprehensive Income) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on available for sale securities	$ (7,590)	$ (9,069)	$ (22,828)
Net unrealized gains (losses) on derivative instruments	181	825	(3,136)
Other comprehensive loss of subsidiaries and associated companies	256,549	208,240	182,884
Other comprehensive income (loss), net of tax	1,110	1,113	(21,845)
Parent Company [Member]
Condensed Statement of Income Captions [Line Items]
Net income	255,439	207,127	204,729
Other comprehensive income (loss), net of tax:
Net unrealized gains (losses) on available for sale securities	0	584	(2,109)
Net unrealized gains (losses) on derivative instruments	1,216	1,223	1,223
Other comprehensive loss of subsidiaries and associated companies	(106)	(694)	(20,959)
Other comprehensive income (loss), net of tax	1,110	1,113	(21,845)
Comprehensive income	$ 256,549	$ 208,240	$ 182,884