Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		$ 4,114,284	$ 3,670,261	$ 3,841,951	$ 3,427,741
Cumulative effect of change in accounting principle related to marketable equity securities	[1]					$ (160)
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate	[2]			0
Other comprehensive (loss) income		(18,369)	1,964	(46,046)	15,929
Ending balance		4,244,850	3,781,896	4,244,850	3,781,896
Available- for-Sale Investment Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(64,822)		(30,898)
Net unrealized (losses) gains arising during the period		(13,584)		(39,588)
Amounts reclassified from AOCI		(24)		(1,673)
Other comprehensive (loss) income		(13,608)		(41,261)
Ending balance		(78,430)		(78,430)
Available- for-Sale Investment Securities | ASU 2016-01
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of change in accounting principle related to marketable equity securities						385
Available- for-Sale Investment Securities | ASU 2018-02
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate				(6,656)
Available- for-Sale Investment Securities | Balance, adjusted
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance				(37,169)
Available- for-Sale Investment Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance			(18,950)		(28,772)
Net unrealized (losses) gains arising during the period			(1,014)		11,818
Amounts reclassified from AOCI			(892)		(3,902)
Other comprehensive (loss) income			(1,906)		7,916
Ending balance			(20,856)		(20,856)
Available- for-Sale Investment Securities | Balance, adjusted
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance					(28,772)
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(6,645)	(15,231)	(6,621)	(19,374)
Net unrealized (losses) gains arising during the period		(4,761)	3,870	(4,785)	8,013
Amounts reclassified from AOCI		0	0	0	0
Other comprehensive (loss) income		(4,761)	3,870	(4,785)	8,013
Ending balance		(11,406)	(11,361)	(11,406)	(11,361)
Foreign Currency Translation Adjustments | Balance, adjusted
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance				(6,621)	(19,374)
AOCI, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(71,467)	(34,181)	(37,519)	(48,146)
Cumulative effect of change in accounting principle related to marketable equity securities	[1]					385
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate	[2]			(6,656)
Net unrealized (losses) gains arising during the period		(18,345)	2,856	(44,373)	19,831
Amounts reclassified from AOCI		(24)	(892)	(1,673)	(3,902)
Other comprehensive (loss) income		(18,369)	1,964	(46,046)	15,929
Ending balance		$ (89,836)	$ (32,217)	(89,836)	(32,217)
AOCI, Net of Tax | ASU 2016-01
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Cumulative effect of change in accounting principle related to marketable equity securities						$ 385
AOCI, Net of Tax | ASU 2018-02
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification of tax effects in AOCI resulting from the new federal corporate income tax rate				(6,656)
AOCI, Net of Tax | Balance, adjusted
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance				$ (43,790)	$ (48,146)

[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.