Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		$ 3,841,951	$ 3,427,741
Cumulative effect of change in accounting principle related to marketable equity securities	[1]			$ (160)
Net unrealized (losses) gains arising during the period		(10,513)	6,062
Amounts reclassified from AOCI		(1,501)	(1,434)
Other comprehensive (loss) income		(12,014)	4,628
Ending balance		3,978,755	3,565,954
Available- for-Sale Investment Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(30,898)	(28,772)
Net unrealized (losses) gains arising during the period		(17,311)	5,055
Amounts reclassified from AOCI		(1,501)	(1,434)
Other comprehensive (loss) income		(18,812)	3,621
Ending balance		(55,981)	(25,151)
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)
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(6,621)	(19,374)
Net unrealized (losses) gains arising during the period		6,798	1,007
Amounts reclassified from AOCI		0	0
Other comprehensive (loss) income		6,798	1,007
Ending balance		177	(18,367)
AOCI, Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance		(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)
Other comprehensive (loss) income		(12,014)	4,628
Ending balance		(55,804)	$ (43,518)
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)

[1] Represents the impact of the adoption in the first quarter of 2018 of Accounting Standards Update (“ASU”) 2016-01, Financial Instruments — Overall (Subtopic 825-10): Recognition and Measurement of Financial Assets and Financial Liabilities. 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 during the first quarter of 2018. Refer to Note 2 — Current Accounting Developments to the Consolidated Financial Statements for additional information.