Income Taxes (Details 2) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of the provision for income taxes, with amounts determined by applying the statutory federal income tax rate to pre-tax income
Income tax at statutory rate		$ 110,493	$ 112,032	$ 93,187
Change in tax rate		10,095
Tax-exempt interest		(2,278)	(2,270)	(2,285)
Other, net		386	(37)	669
Income tax expense		118,696	$ 109,725	$ 91,571
Effect of Tax Cuts and Jobs Act of 2017
Increase in income tax expense		10,095
Tax Cuts and Jobs Act of 2017
Effect of Tax Cuts and Jobs Act of 2017
Increase in income tax expense	$ 10,100	10,100
Increase in income tax expense due to re-measurement of net deferred tax assets		19,900
Deferred tax benefit related to acceleration of pension contributions		$ (9,800)