Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Consolidated Statements of Comprehensive Income [Abstract]
Net income	$ 46,907	$ 34,829	$ 50,746	$ 74,988
Other comprehensive income:
Net change in unrealized gain (loss)	16,629	(18,370)	45,601	(12,957)
Reclassification adjustment for net losses realized and included in earnings	711	801	1,802	1,406
Valuation adjustment for employee benefit plans		(5,922)		(5,922)
Amortization of unrealized net gain on securities transferred to held to maturity	830	939	1,628	1,586
Other comprehensive income (loss) before income taxes	18,170	(22,552)	49,031	(15,887)
Income tax	6,645	(8,237)	17,977	(5,877)
Other comprehensive income (loss) net of income taxes	11,525	(14,315)	31,054	(10,010)
Comprehensive income	$ 58,432	$ 20,514	$ 81,800	$ 64,978