Consolidated Statement of Changes in Stockholders' Equity - USD ($) $ in Thousands		Total	Common Stock	Common Stock and Additional Paid-In Capital	Retained Earnings	Treasury Stock	AOCI, Net of Tax
Beginning balance (in shares) at Dec. 31, 2016			144,167,451
Beginning balance at Dec. 31, 2016		$ 3,427,741		$ 1,727,598	$ 2,187,676	$ (439,387)	$ (48,146)
Increase (Decrease) in Stockholders' Equity
Net income		420,726			420,726
Other comprehensive income (loss)		15,929					15,929
Stock compensation costs		15,780		15,780
Net activity of common stock pursuant to various stock compensation plans and agreements (in shares)			343,492
Net activity of common stock pursuant to various stock compensation plans and agreements		(10,695)		1,968
Net activity of common stock pursuant to various stock compensation plans and agreements						(12,663)
Cash dividends on common stock		(87,585)			(87,585)
Ending balance (in shares) at Sep. 30, 2017			144,510,943
Ending balance at Sep. 30, 2017		3,781,896		1,745,346	2,520,817	(452,050)	(32,217)
Increase (Decrease) in Stockholders' Equity
Dividends declared per common share (in dollars per share)			$ 0.60
Beginning balance (in shares) at Jun. 30, 2017			144,486,326
Beginning balance at Jun. 30, 2017		3,670,261		1,738,721	2,417,367	(451,646)	(34,181)
Increase (Decrease) in Stockholders' Equity
Net income		132,660			132,660
Other comprehensive income (loss)		1,964					1,964
Stock compensation costs		5,665		5,665
Net activity of common stock pursuant to various stock compensation plans and agreements (in shares)			24,617
Net activity of common stock pursuant to various stock compensation plans and agreements		556		960
Net activity of common stock pursuant to various stock compensation plans and agreements						(404)
Cash dividends on common stock		(29,210)			(29,210)
Ending balance (in shares) at Sep. 30, 2017			144,510,943
Ending balance at Sep. 30, 2017		3,781,896		1,745,346	2,520,817	(452,050)	(32,217)
Increase (Decrease) in Stockholders' Equity
Dividends declared per common share (in dollars per share)			$ 0.20
Cumulative effect of change in accounting principle related to marketable equity securities	[1]	(160)			(545)		385
Beginning balance (in shares) at Dec. 31, 2017			144,543,060
Beginning balance at Dec. 31, 2017		3,841,951		1,755,495	2,576,302	(452,327)	(37,519)
Increase (Decrease) in Stockholders' Equity
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate	[2]	0			6,656		(6,656)
Net income		530,683			530,683
Other comprehensive income (loss)		(46,046)					(46,046)
Stock compensation costs		24,201		24,201
Net activity of common stock pursuant to various stock compensation plans and agreements (in shares)			386,371
Net activity of common stock pursuant to various stock compensation plans and agreements		(13,475)		(13,475)
Cash dividends on common stock		(92,304)			(92,304)
Ending balance (in shares) at Sep. 30, 2018			144,929,431
Ending balance at Sep. 30, 2018		4,244,850		1,766,221	3,020,792	(452,327)	(89,836)
Increase (Decrease) in Stockholders' Equity
Dividends declared per common share (in dollars per share)			$ 0.63
Beginning balance (in shares) at Jun. 30, 2018			144,904,629
Beginning balance at Jun. 30, 2018		4,114,284		1,754,877	2,883,201	(452,327)	(71,467)
Increase (Decrease) in Stockholders' Equity
Net income		171,302			171,302
Other comprehensive income (loss)		(18,369)					(18,369)
Stock compensation costs		10,986		10,986
Net activity of common stock pursuant to various stock compensation plans and agreements (in shares)			24,802
Net activity of common stock pursuant to various stock compensation plans and agreements		358		358
Cash dividends on common stock		(33,711)			(33,711)
Ending balance (in shares) at Sep. 30, 2018			144,929,431
Ending balance at Sep. 30, 2018		$ 4,244,850		$ 1,766,221	$ 3,020,792	$ (452,327)	$ (89,836)
Increase (Decrease) in Stockholders' Equity
Dividends declared per common share (in dollars per share)			$ 0.23

[1] Represents the impact of the adoption of Accounting Standards Update (“ASU”) 2016-01, Financial Instruments — Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities in the first quarter of 2018. Refer to Note 2 — Current Accounting Developments to the Consolidated Financial Statements for additional information.

[2] Represents amounts reclassified from AOCI to retained earnings due to the early adoption of ASU 2018-02, Income Statement — Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income in the first quarter of 2018. Refer to Note 2 — Current Accounting Developments to the Consolidated Financial Statements for additional information.