Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 117,791	$ 94,031	$ 223,496	$ 177,762
Net income, Tax (Expense) Benefit	(41,788)	(33,574)	(75,635)	(64,773)
Net income	76,003	60,457	147,861	112,989
Reclassification adjustment for losses realized in net income, Before-tax Amount	65	64	130	337
Reclassification adjustment for losses realized in net income, Tax (Expense) Benefit	(25)	(25)	(50)	(130)
Reclassification adjustment for losses realized in net income, Net of Tax Amount	40	39	80	207
Net unrealized gains on investment securities available for sale:
Reclassification adjustment for net (gains) losses realized in net income, Before-tax Amount	1	0	(7,667)	(67)
Net unrealized gains arising during the period, Before-tax Amount	11,150	19,044	20,250	66,215
Net unrealized gain, Before-tax Amount	11,151	19,044	12,583	66,148
Net unrealized gains on investment securities available for sale: Tax (Expense) Benefit
Reclassification adjustment for net (gains) losses realized in net income, Tax (Expense) Benefit	0	0	2,952	26
Net unrealized gains arising during the period, Tax (Expense) Benefit	(4,293)	(7,332)	(7,797)	(25,493)
Net unrealized gains, Tax (Expense) Benefit	(4,293)	(7,332)	(4,845)	(25,467)
Net unrealized gains on investment securities available for sale: Net of Tax Amount
Reclassification adjustment for net (gains) losses realized in net income, Net of Tax Amount	1	0	(4,715)	(41)
Net unrealized gains arising during the period, Net of Tax Amount	6,857	11,712	12,453	40,722
Net unrealized gains, Net of Tax Amount	6,858	11,712	7,738	40,681
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for gains realized in net income, Before-tax Amount	(20)	(10)	(40)	(104)
Actuarial gains arising during the period, Before-tax Amount	0	0	0	0
Net unrealized (realized) gains, Before-tax Amount	(20)	(10)	(40)	(104)
Post-retirement unfunded health benefit: Tax (Expense) Benefit
Reclassification adjustment for gains realized in net income, Tax (Expense) Benefit	8	4	16	40
Actuarial gains arising during the period, Tax (Expense) Benefit	0	0	0	0
Net unrealized (realized) gains, Tax (Expense) Benefit	8	4	16	40
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for gains realized in net income, Net of Tax Amount	(12)	(6)	(24)	(64)
Actuarial gains arising during the period, Net of Tax Amount	0	0	0	0
Net unrealized (realized) gains, Net of Tax Amount	(12)	(6)	(24)	(64)
Other comprehensive income, Before-tax Amount	11,196	19,098	12,673	66,381
Other comprehensive income, Tax (Expense) Benefit	(4,310)	(7,353)	(4,879)	(25,557)
Other comprehensive income, Net of Tax Amount	6,886	11,745	7,794	40,824
Comprehensive income, Net of Tax Amount	$ 82,889	$ 72,202	$ 155,655	$ 153,813