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		288,066			288,066
Other comprehensive income (loss)		13,965					13,965
Stock compensation costs		10,115		10,115
Net activity of common stock pursuant to various stock compensation plans and agreements (in shares)			318,875
Net activity of common stock pursuant to various stock compensation plans and agreements		(11,251)		1,008
Net activity of common stock pursuant to various stock compensation plans and agreements						(12,259)
Cash dividends on common stock		(58,375)			(58,375)
Ending balance (in shares) at Jun. 30, 2017			144,486,326
Ending balance at Jun. 30, 2017		3,670,261		1,738,721	2,417,367	(451,646)	(34,181)
Beginning balance at Mar. 31, 2017							(43,518)
Increase (Decrease) in Stockholders' Equity
Net income		118,330
Other comprehensive income (loss)		9,337					9,337
Ending balance (in shares) at Jun. 30, 2017			144,486,326
Ending balance at Jun. 30, 2017		3,670,261		1,738,721	2,417,367	(451,646)	(34,181)
Increase (Decrease) in Stockholders' Equity
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		359,381			359,381
Other comprehensive income (loss)		(27,677)					(27,677)
Stock compensation costs		13,215		13,215
Net activity of common stock pursuant to various stock compensation plans and agreements (in shares)			361,569
Net activity of common stock pursuant to various stock compensation plans and agreements		(13,833)		(13,833)
Net activity of common stock pursuant to various stock compensation plans and agreements						0
Cash dividends on common stock		(58,593)			(58,593)
Ending balance (in shares) at Jun. 30, 2018			144,904,629
Ending balance at Jun. 30, 2018		4,114,284		1,754,877	2,883,201	(452,327)	(71,467)
Beginning balance at Mar. 31, 2018							(55,804)
Increase (Decrease) in Stockholders' Equity
Net income		172,349
Other comprehensive income (loss)		(15,663)					(15,663)
Ending balance (in shares) at Jun. 30, 2018			144,904,629
Ending balance at Jun. 30, 2018		$ 4,114,284		$ 1,754,877	$ 2,883,201	$ (452,327)	$ (71,467)

[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.