Significant Accounting Policies (Reclassifications Out of Accumulated Other Comprehensive Income) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income from operations before income taxes		$ 132,102	$ 98,802	$ 199,811	$ 160,762
Tax expense (benefit)		73,734	(22,065)	111,455	11,108
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Net of tax		600	1,200	600	1,300
Reclassification out of Accumulated Other Comprehensive Income [Member] | Unrealized gains (losses) on investments available-for-sale [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Unrealized gains (losses) on investments available-for-sale		1,600	1,900	2,200	2,200
Income from operations before income taxes		1,600	1,900	2,200	2,200
Tax expense (benefit)		600	600	800	700
Net of tax		1,000	1,300	1,400	1,500
Reclassification out of Accumulated Other Comprehensive Income [Member] | Amortization of defined benefit pension items [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Prior-service cost	[1]	(100)	(100)	(200)	(200)
Actuarial gains (losses)	[1]	(600)	0	(1,100)	(100)
Income from operations before income taxes		(700)	(100)	(1,300)	(300)
Tax expense (benefit)		(300)	0	(500)	(100)
Net of tax		$ (400)	$ (100)	$ (800)	$ (200)

[1] These accumulated other comprehensive income components are included in the computation of net periodic pension cost.