Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands		Total	Directors' stock plans	Stock options	Restricted stock units	Restricted stock awards	Common stock	Common stock Directors' stock plans	Common stock Stock options	Common stock Restricted stock units	Common stock Restricted stock awards	Additional paid-in capital	Additional paid-in capital Directors' stock plans	Additional paid-in capital Stock options	Additional paid-in capital Restricted stock units	Additional paid-in capital Restricted stock awards	Retained earnings	Accumulated other comprehensive income (loss)
Beginning balance at Dec. 31, 2015		$ 1,015,974					$ 38,168					$ 725,280					$ 281,558	$ (29,032)
Changes in Stockholders' Equity
Comprehensive income		52,714															49,380	3,334
Cash dividends declared and paid, $0.54 and $0.52 per share at June 30, 2017 and June 30, 2016, respectively		(19,942)															(19,942)
Shares issued under share-based compensation			$ 125	$ 400				$ 5	$ 48				$ 120	$ 352
Net shares issued - restricted stock units/awards					$ (752)					$ 45					$ (797)
Share-based compensation expense		1,780					1					1,779
Ending balance at Jun. 30, 2016		1,050,299					38,267					726,734					310,996	(25,698)
Beginning balance at Mar. 31, 2016																		(26,690)
Changes in Stockholders' Equity
Comprehensive income		26,767
Ending balance at Jun. 30, 2016		1,050,299					38,267					726,734					310,996	(25,698)
Changes in Stockholders' Equity
Cumulative effect adjustment of change in accounting policy, net of tax impact	[1]	3,659															3,659
Beginning balance at Dec. 31, 2016		2,581,526					70,599					2,210,762					340,201	(40,036)
Changes in Stockholders' Equity
Comprehensive income		105,525															99,618	5,907
Cash dividends declared and paid, $0.54 and $0.52 per share at June 30, 2017 and June 30, 2016, respectively		(38,539)															(38,539)
Shares issued under share-based compensation				$ (17,078)					$ 523					$ (17,601)
Net shares issued - restricted stock units/awards					$ (1,296)	$ (1,283)				$ 35	$ (27)				$ (1,331)	$ (1,256)
Share-based compensation expense		6,928					1					6,927
Ending balance at Jun. 30, 2017		2,639,442					71,131					2,197,501					404,939	(34,129)
Beginning balance at Mar. 31, 2017																		(37,965)
Changes in Stockholders' Equity
Comprehensive income		55,850
Ending balance at Jun. 30, 2017		$ 2,639,442					$ 71,131					$ 2,197,501					$ 404,939	$ (34,129)

[1] Refer to Note 1, Basis of Presentation and Significant Accounting Policies and Note 8, Loan Servicing Rights, for further details on the changes in accounting policy.