Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income, Before-tax Amount	$ 480,138	$ 388,451	$ 358,573
Net income, Tax (Expense) Benefit	(204,664)	(141,667)	(132,491)
Net income	275,474	246,784	226,082
Net change related to cash flow hedges: Before-tax Amount
Reclassification adjustment for losses realized in net income, Before-tax Amount	130	467	521
Net change related to cash flow hedges: Tax (Expense) Benefit
Reclassification adjustment for losses realized in net income, Tax (Expense) Benefit	(50)	(180)	(201)
Net change related to cash flow hedges: Net of Tax Amount
Reclassification adjustment for losses realized in net income, Net of Tax Amount	80	287	320
Net unrealized gains (losses) on investment securities available for sale: Before-tax Amount
Reclassification adjustment for net losses (gains) realized in net income, Before-tax Amount	289	(6,011)	(2,769)
Net unrealized gains (losses) arising during the period, Before-tax Amount	1,038	(36,432)	(25,707)
Net unrealized gains (losses), Before-tax Amount	1,327	(42,443)	(28,476)
Net unrealized gains (losses) on investment securities available for sale: Tax (Expense) Benefit
Reclassification adjustment for net losses (gains) realized in net income, Tax (Expense) Benefit	(111)	2,314	1,066
Net unrealized gains (losses) arising during the period, Tax (Expense) Benefit	(362)	14,027	9,901
Net unrealized gains (losses), Tax (Expense) Benefit	(473)	16,341	10,967
Net unrealized gains (losses) on investment securities available for sale: Net of Tax Amount
Reclassification adjustment for net losses (gains) realized in net income, Net of Tax Amount	178	(3,697)	(1,703)
Net unrealized gains (losses) arising during the period, Net of Tax Amount	676	(22,405)	(15,806)
Net unrealized gains (losses), Net of Tax Amount	854	(26,102)	(17,509)
Post-retirement unfunded health benefit: Before-tax Amount
Reclassification adjustment for gains realized in net income, Before-tax Amount	(110)	(144)	(272)
Actuarial gains arising during the period, Before-tax Amount	61	102	236
Net unrealized (losses) gains, Before-tax Amount	(49)	(42)	(36)
Post-retirement unfunded health benefit: Tax (Expense) Benefit
Reclassification adjustment for gains realized in net income, Tax (Expense) Benefit	43	56	104
Actuarial gains arising during the period, Tax (Expense) Benefit	(23)	(39)	(93)
Net unrealized (losses) gains, Tax (Expense) Benefit	20	17	11
Post-retirement unfunded health benefit: Net of Tax Amount
Reclassification adjustment for gains realized in net income, Net of Tax Amount	(67)	(88)	(168)
Actuarial gains arising during the period, Net of Tax Amount	38	63	143
Net unrealized (losses) gains, Net of Tax Amount	(29)	(25)	(25)
Other comprehensive income (loss), Before-tax Amount	1,408	(42,018)	(27,991)
Other comprehensive income (loss), Tax (Expense) Benefit	(503)	16,178	10,777
Net current period other comprehensive income (loss)	905	(25,840)	(17,214)
Comprehensive income, Net of Tax Amount	$ 276,379	$ 220,944	$ 208,868